UNIVERSIDADE FEDERAL DE SANTA CATARINA CAMPUS FLORIANÓPOLIS
PROGRAMA DE PÓS-GRADUAÇÃO EM CIÊNCIA E ENGENHARIA DE MATERIAIS
Tobias Benitez
Processamento de vitrocerâmicas transparentes com alta resistência ao impacto
Florianópolis/Erlangen 2020
Tobias Benitez
Processamento de vitrocerâmicas transparentes com alta resistência ao impacto
Tese submetida ao Programa de Pós-Graduação em Ciência e Engenharia de Materiais da Universidade Federal de Santa Catarina e pela Friedrich-Alexander Universität Erlangen-Nürnberg em regime de cotutela para a obtenção do título de Doutor em Ciência e Engenharia de Materiais.
Orientadores (UFSC): Prof. Dr. Dachamir Hotza
Prof. Dr. Antonio Pedro Novaes de Oliveira Supervisor (FAU/Alemanha):
Prof. Dr. Nahum Travitzky
Florianópolis/Erlangen 2020
Tobias Benitez
Processing of Transparent Glass-Ceramics with Enhanced Impact Resistance
Cotutelle thesis presented to the Graduate Program in Materials Science and Engineering of the Federal University of Santa Catarina and the Friedrich-Alexander Universität Erlangen-Nürnberg as a requirement to obtain the PhD title.
Advisors (UFSC): Prof. Dr. Dachamir Hotza
Prof. Dr. Antonio Pedro Novaes de Oliveira Supervisor (FAU/Germany):
Prof. Dr. Nahum Travitzky
Florianópolis/Erlangen 2020
Tobias Benitez
Processamento de Vitrocerâmicas Transparentes com Alta Resistência ao Impacto
O presente trabalho em nível de doutorado foi avaliado e aprovado por banca examinadora composta pelos seguintes membros:
Prof. Dr. Nahum Travitzky
Friedrich-Alexander Universität Erlangen-Nürnberg
Prof. Dr. Dominique de Ligny
Friedrich-Alexander Universität Erlangen-Nürnberg
Prof. Dr. Carlos Renato Rambo Universidade Federal de Santa Catarina
Prof. Dr. Guilherme Mariz de Oliveira Barra Universidade Federal de Santa Catarina
Certificamos que esta é a versão original e final do trabalho de conclusão que foi julgado adequada para obtenção do título de doutor em Ciência e Engenharia de Materiais.
____________________________ Prof. Dr. Guilherme Mariz de Oliveira Barra
Coordenador do Programa
____________________________ Prof. Dr. Dachamir Hotza
Orientador
Florianópolis/Erlangen, 2020.
Dachamir
Hotza:49523503987
Digitally signed by Dachamir Hotza:49523503987
Date: 2020.03.09 11:28:05 -03'00'
Assinado de forma digital por Guilherme Mariz de Oliveira Barra:17871842854 Dados: 2020.03.11 14:30:31 -03'00'
ACKNOWLEDGMENTS
My biggest and sincere “thank you” goes to my advisors Prof. Dachamir Hotza, Prof. Nahum Travitzky and Prof. Antonio Pedro Novaes de Oliveira. Their support and experience were vital for this project.
Thanks to Prof. Dominique de Ligny for his support and advices during my time at FAU. The LabMat/UFSC staff who embraced me as one of their team at the beginning of the project is gratefully acknowledged, as well as Prof. Sergio Gómez and my undergraduate assistant Julia Barbetta for her support and dedication.
Thanks to Dr. Diego Blaese, Dr. Kaline Furlan, Dr. Hans Jelitto and Dr. Rolf Janssen during my time at TUHH.
A special acknowledgment to Dr. Sandro Rivas, who was crucial for simulations. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nivel Superior – Brasil (CAPES) – Project N° CAPES-PRINT/88881.310728/2018-01, as well as, the German DAAD and Colombian COLFUTURO agencies.
RESUMO
Cerâmicas e vitrocerâmicas transparentes usadas para aplicações de resistência ao impacto apresentam limitações – como os altos custos de produção, entre outros desafios – que são discutidos na presente tese. A informação relacionada ao desenho dos sistemas de blindagem transparente é bastante dispersa e, de alguma forma, escassa, o que dificulta o entendimento e a solução dos desafios mencionados anteriormente. Por tanto, nessa tese apresenta-se o panorama dos materiais para blindagem transparente, incluindo informações sobre estrutura dos materiais, fabricação e propriedades, as quais podem ser úteis para interessados em entrar nessa área de pesquisa. O estado atual da técnica – incluindo desafios, lacunas tecnológicas, limitações – e os mais recentes achados científicos indicam que existem possibilidades para a fabricação de baixo custo de materiais para os sistemas de blindagem transparentes com maior desempenho. Assim, vidros de aluminossilicato, xAl2O3·(1-x)SiO2, com razão molar variável
0 ≤ x ≤ 0,6, foram simulados por dinâmica molecular a fim de investigar as características estruturais responsáveis for sua performance mecânica. Testes de tensão foram realizados na dinâmica molecular para estudar os danos em escala nanométrica. As propriedades mecânicas simuladas foram associadas ao desempenho dos sistemas de blindagem transparente. Adicionalmente, uma vitrocerâmica de magnésio-aluminossilicato nucleada com Nb2O5 foi
fabricada, caracterizada e os resultados discutidos. A influência dos agentes de nucleação nos sistemas foi extensamente analisada. Por fim, essa tese apresenta conclusões gerais e sugere próximos passos na pesquisa nesta área.
Palavras-Chaves: Aluminossilicato. Vitrocerâmicas. Dinâmica Molecular. Blindagem. Cristalização. Resistência ao Impacto.
RESUMO EXPANDIDO Introdução
A vida é um direito fundamental dos seres humanos. Ao longo da história, várias conquistas da ciência foram aplicadas na seleção e/ou fabricação de materiais tecnológicos para a proteção da vida. Isso acontece desde dos tempos nos quais se fabricavam armaduras utilizando peles de animais, até os dias atuais como, por exemplo, com a produção dos sistemas de blindagem transparentes. Esses sistemas têm quatro seções: frontal, intermediária, traseira e adesivos. Cada uma dessas partes tem uma função específica; no entanto, a que tem maior influência na resistência dos impactos é a sessão frontal. Assim, esta pesquisa se focou na fabricação de vitrocerâmicas transparentes para a sessão frontal.
Inicialmente, foi realizada uma revisão bibliográfica e provas preliminares com cerâmicas e vitrocerâmicas. No entanto, se tomou a decisão de se focar as vitrocerâmicas transparentes, já que para este material há maiores chances de produção em escala industrial quando comparado às cerâmicas. A escolha da composição química do vidro precursor, da estequiometria e do agente de nucleação foram baseados em trabalhos prévios de outros autores. Além disso, nesta tese se iniciou a construção de uma ferramenta computacional robusta para a seleção dos materiais mais indicados para determinadas aplicações. O desenho da microestrutura e a predição das propriedades mecânicas através de simulações por computador pode fazer aumentar a eficiência da busca pelo material ideal. Uma vez que se realize esse processo de simulação, deve-se verificar a veracidade das informações por meio de ensaios práticos. No presente trabalho, esses ensaios foram realizados no sistema magnésio-alumínio-silicato, nucleado com nióbia (Nb2O5).
Objetivos
O objetivo geral desta tese é desenvolver uma sessão frontal transparente de vitrocerâmica de magnésio-alumínio-silicato para sistemas de blindagens transparentes com equilíbrio entre resistência a impactos e viabilidade industrial. Além do objetivo geral, o trabalho tem três objetivos específicos: (i) revisar o estado da técnica dos materiais frágeis transparentes usados em sistemas de blindagens, para identificar famílias com potencial para realçar a performance desses sistemas; (ii) simular com dinâmicas moleculares, a estrutura e as propriedades mecânicas dos vidros alumínio-silicatos, xAl2O3·(1-x)SiO2 com variável molar de 0 ≤ x ≤ 0,6;
(iii) avaliar a influência dos agentes de nucleação na cinética de cristalização e propriedades mecânicas das vitrocerâmicas de magnésio-alumínio-silicato.
Metodologia
A revisão bibliográfica correspondeu à publicação de um artigo com o resumo do estado da técnica das cerâmicas e vitrocerâmicas transparentes para os sistemas de blindagens transparentes. Escolheu-se o magnésio-alumino-silicato com estequiometria no ponto eutéetico da cordierita e a enstatita como vidro precursor, de maneira a reduzir a temperatura de fusão do líquido. Adicionalmente, os agentes de nucleação tradicionais, TiO2 e ZrO2,
foram comparados com outros de maior ponto de fusão e maior energia livre de Gibbs, MoO3
e Nb2O5. A caracterização estrutural, mecânica e óptica desses materiais constitui a parte
experimental desse trabalho de pesquisa. A metodologia para a seleção dos tempos e temperaturas de nucleação e cristalização foi baseada na análise térmica das amostras avaliadas. Em paralelo, a simulação computacional usando dinâmica molecular de vidros alumínio-silicato foi realizada; Com essas simulações, foi possível prever a resposta da sílica pura com a adição de alumina quando testes de tensão unidimensional são realizados.
Resultados e discussão
A sílica pura foi simulada usando dinâmica molecular; posteriormente, foi adicionada progressivamente alumina até atingir 60% do conteúdo. Amostras de cada uma das composições simuladas foram testadas sob tensão uniaxial no computador. Assim, foi possível estimar que a quantidade de átomos de oxigênio com número de coordenação 2 é reduzido de 100% até 50% com o aumento do conteúdo da alumina na amostra. O oposto aconteceu com os átomos de oxigênio com número de coordenação 3, que foram desde 0% na sílica pura até 50% com a maior porcentagem de alumina adicionada. Os átomos de alumínio com número de coordenação 4 foram de 85% no vidro 0.3Al2O3·0.7SiO2 até 60% no vidro
0.6Al2O3·0.4SiO2. No entanto, os átomos de alumínio com número de coordenação 5
aumentaram de 15% até 35%. A função de distribuição de pares de átomos e do ângulo, do inglês PDF e ADF, respectivamente, confirmaram que a adição de alumina produz densificação da sílica pura, o que é coerente com resultados experimentais de vidros alumínio-silicato. A simulação do teste de tensão mostrou que os vidros alumínio-silicato apresentam uma região elástica menor que a sílica pura, no entanto, os primeiros são capazes de atingir maiores deformações que a sílica pura sem quebrar completamente.
Na parte experimental dessa pesquisa, foi adicionado magnésio, como modificador da rede dos vidros alumínio-silicato, e pentóxido de nióbio (Nb2O5), como agente de nucleação. Os
resultados mostraram que o tratamento térmico para produção de vitrocerâmicas para serem usadas na capa frontal precisam temperaturas de 880 °C e tempos maiores de 120 min. As influências da temperatura e do tempo de cristalização nas propriedades mecânicas finais do material são maiores que a influência do processo de nucleação. Foi possível atingir uma fração volumétrica cristalina de até 20% com uma transmitância de 70%. A dureza Knoop subiu 20% em comparação com o vidro precursor.
Considerações finais
As vitrocerâmicas são candidatas a serem usadas na sessão frontal dos sistemas de blindagens transparentes. No entanto, a composição química do vidro precursor ideal ainda é indefinida. Simulações e outros métodos de desenho computacional são ferramentas úteis que podem aumentar a eficiência no desenvolvimento de vitrocerâmicas transparentes. Diferentes composições e tamanhos foram sugeridos, de modo que a formação e o crescimento de trincas podem aparecer depois de vários ciclos de simulação. Em conclusão, o desenvolvimento da vitrocerâmica ideal para ser aplicada em blindagens transparentes é ainda um desafio.
Palavras-Chaves: Alumínio-silicato. Vitro-cerâmicas. Dinâmica Molecular. Blindagem. Cristalização. Resistência ao Impacto.
ABSTRACT
Transparent ceramics and glass-ceramics intended for impact-resistant applications present limitations – such as high production cost, among other challenges – as discussed in this thesis. The information concerning transparent systems design is highly scattered, and to some extent scarce in specific points, scenario that makes difficult to understand and cope the abovementioned challenges. Therefore, we intend to simplify the landscape of transparent impact-resistant materials and give some tutorial information about materials structure, fabrication and properties, which may be useful for those interested in entering the field. The state of the technique – including current challenges, technological gaps, limitations – and the most recent findings show the further field development towards simplified and low-cost fabrication of materials for transparent systems. Indeed, aluminosilicate glasses, xAl2O3
·(1-x)SiO2 with variable molar ratio 0 ≤ x ≤ 0.6, are modeled by molecular dynamics in order to
investigate the structural characteristics responsible for their mechanical performance. Tension tests were simulated in molecular dynamics to study the mechanisms of structural damage at nanoscale. The mechanical properties – estimated by molecular dynamics – are linked to macroscale properties influencing transparent armor systems’ performance. Additionally, a magnesium-aluminum-silicate glass-ceramic nucleated with Nb2O5 is
fabricated, characterized and the respective results are discussed. The influence of nucleating agents in the magnesium-aluminum-silicate systems is extensively analyzed. Finally, this thesis gives general conclusions and suggests the next steps in this research field.
Key words: Aluminosilicates. Glass-Ceramics. Molecular Dynamics. Crystallization. Impact Resistance.
KURZFASSUNG
Transparente Keramik und Glaskeramik für Schlagfestigkeitsanwendungen weisen Einschränkungen auf, wie z. B. hohe Produktionskosten und andere Herausforderungen, die im ersten Kapitel dieses Dokuments diskutiert werden. Die Informationen zum Design transparenter systeme sind stark verteilt und in einigen Fällen knapp, was es schwierig macht, die oben genannten Herausforderungen zu verstehen und zu bewältigen. Aus diesem Grund möchten wir im zweiten Kapitel die Landschaft der transparenten Schlagfestigkeitsmaterialien vereinfachen und einige TutorialInformationen zur Materialstruktur, herstellung und -eigenschaften bereitstellen, die für Interessenten nützlich sein können. Der Stand der Technik – einschließlich aktueller Herausforderungen, technologischer Lücken, Grenzen – und die neuesten Erkenntnisse zeigen die weitere Feldentwicklung hin zu einer vereinfachten und kostengünstigen Produktion. Im dritten Kapitel dieser Arbeit werden Aluminosilikatgläser, xAl2O3·(1-x)SiO2 mit einem variablen Molverhältnis von 0 ≤ x ≤ 0,6, molekulardynamisch
modelliert, um die strukturellen Eigenschaften zu untersuchen, die für das mechanische Verhalten verantwortlich sind. Stresstests wurden in der Molekulardynamik simuliert, um die Mechanismen von nanoskaligen Strukturschäden zu untersuchen. Die durch die Molekulardynamik geschätzten mechanischen Eigenschaften stehen in Zusammenhang mit makroskaligen Eigenschaften, die die Leistung transparenter systeme beeinflussen. Im vierten Kapitel wird eine Nb2O5-kernhaltige Magnesium-Aluminium-Silikat-Glaskeramik hergestellt,
charakterisiert und die jeweiligen Ergebnisse werden diskutiert. Der Einfluss von Keimbildnern in den Magnesium-Aluminium-Silikat-Systemen wird im Detail analysiert. Das fünfte Kapitel dieser Arbeit befasst sich mit der Messung der R-Kurve. Dort wurde eine neuartige Methode zur Messung der Bruchzähigkeit verwendet, um einige Bruchmechanismen zu untersuchen, die in transparenten Schlagfestigkeitsystemen vorhanden sein können. Das letzte Kapitel der Arbeit gibt allgemeine Schlussfolgerungen und schlägt die nächsten Schritte in diesem Forschungsbereich vor.
Schlüsselwörter: Aluminosilicate. Glaskeramik. Molekulare Dynamik. Kristallisation. Schlagfestigkeit.
TABLE OF FIGURES
Figure 1 - Functional layers on transparent armor concept design. ... 26
Figure 2 - Schematic light transmission phenomena in a polycrystalline ceramic. ... 29
Figure 3 - Light transmission of selected ceramics and glass-ceramics. ... 29
Figure 4 - PMAS hardness results scattering and possible Hall-Petch relationship between hardness and nanometric grain size. ... 32
Figure 5 - Correlation between static and dynamic hardness. ... 32
Figure 6 - Suggested key properties ranking hypothesis affecting the armor impact resistance. ... 33
Figure 7 - Fragments of a projectile after perforation of Al2O3 using (a) aluminum and (b) glass backing... 36
Figure 8 - Schematic impact test set up to measure V0 velocity. ... 36
Figure 9 - DOP test set up for armor using polycarbonate as a substrate... 37
Figure 10 - Fragment mode of strike face materials after DOP test. ... 38
Figure 11 - Single-impact resistance of potential strike face materials (a) minimum areal density and strike face required to stop a 7.62 mm 51 AP, steel core projectile and (b) DOP results of PMAS and alumina with glass backing. ... 40
Figure 12 - Transparent armor materials for STANAG 4569: (a) Level 2, and (b) Level 3. ... 42
Figure 13 - Phase diagram of the MgO-Al2O3 system. ... 47
Figure 14 - General and detail view of transparent pieces manufactured by PS/HIP with three commercial available raw materials powders: a) Baikowski, b) Nanocerox and, c) Taimei. .. 50
Figure 15 - HP/HIP & PS/HIP sintering processes of PMAS plates. ... 54
Figure 16 - Pore size distribution of green body’s densities of commercial magnesium aluminate spinel raw materials a) CIP-ed at 200 MPa and slip cast and b) CIP-ed at 350 MPa. ... 56
Figure 17 - Visible light in-line transmission of Taimei TSP-20 powder shaped by slip casting and CIP. ... 58
Figure 18 - Phase diagram of MgO-Al2O3-SiO2 system (a) ternary with red point highlighted the 20MgO-20Al2O3-60SiO2 wt.% composition and (b) a pseudobinary section of the red dotted line with a eutectic point. ... 65
Figure 20 - Structural units observed in the nanostructure of the aluminosilicate samples atoms represented as colored spheres; O atoms are red, Si atoms are yellow and Al atoms are
green. ... 80
Figure 21 - CN for specimens (2940 N + 20 ps) with different chemical compositions... 82
Figure 22 - PDF for specimens (2940 N + 20 ps) with different chemical compositions. ... 85
Figure 23 - ADF for specimens (2940 N + 20 ps) with different chemical compositions. ... 86
Figure 24 - Axial stress versus axial strain for aluminosilicate glass samples. ... 87
Figure 25 - Samples of 100Si-Al and 40Si-60Al glass systems after 50% strain. ... 87
Figure 26 - Bond damage under uniaxial deformation for (2940 N + 20 ps) samples. ... 89
Figure 27 - Bond damage of Al4 and Al5 structures under uniaxial deformation for (2940 N + 20 ps) samples. ... 90
Figure 28 - Bond damage of O2 and O3 structures under uniaxial deformation for (2940 N + 20ps) samples. ... 90
Figure 29 - Appearance of MAS pristine glass (a) and with 1.0 mol% Nb2O5 (b), MoO3 (c) and TiO2+ZrO2 (d). Additionally, MAS with 0.2 mol% of MoO3 melted in reducing condition (e) and GC (f) are presented. The references: (WANG et al., 2013a) precursor glass (g) and GC (h), PMAS (i) and LAS (j) are also shown. ... 91
Figure 30 – DSC and contact dilatometric results of the MAS precursor glass with 1.0 mol% MoO3. Additionally, the nucleation and crystallization temperatures tested in this work are highlighted. ... 92
Figure 31 – Optical dilatometric images of polished MAS precursor glass with 1.0 mol% of MoO3. At 950 °C the sample remains as transparent as the precursor glass, after 3 min (heating rate of 10 °C/min) crystals diminishing transparency starts to appear at surface. The thickness of the crystallized layer growths up to full the volume at 1070 °C. ... 93
Figure 32 – Illustrative two-step heat-treatment cycle (1) for MAS precursor glasses and predicted crystallized fraction when MAS was nucleated with 1.0 mol% of TiO2+ZrO2 (2), MoO3 (4) and Nb2O5 (5). Additionally, MAS with 0.2 mol% of MoO3 melted in reducing condition (3). ... 94
Figure 33 – Predicted crystallized fraction of Nb-MAS and Mo-MAS glass-ceramics according to different heat-treatments conditions. Nb-MAS glass-ceramics produce higher crystallized fraction than Mo-MAS glass-ceramics. ... 95
Figure 34 – Raman spectra of MAS pristine glass (1) and with 1.0 mol% TiO2+ZrO2 (2), MoO3 (3) and Nb2O5 (4). ... 97
Figure 35 – XRD spectra of Nb-MAS precursor glass and glass-ceramics. TN 775 °C, tN 4 h,
TC 910 °C, crystallization times (tC) of 0, 60 and 120 min... 97
Figure 36 – SEM micrographs of Nb-MAS glass-ceramic: cross section (a,b,c) and surface (d) at TN 775 °C, tN 240 min, TC 910 °C and tC 60 min. ... 99
Figure 37 – Influence of thermal treatment on Knoop hardness of Nb-MAS glass-ceramics’. ... 100 Figure 38 – Indentation size effect and Knoop hardness evolution of Nb-MAS CGs and references materials. Several thermal-treatment cycles were performed to the Nb-MAS GC. ... 101 Figure 39 – SEM micrographs of Nb-MAS glass-ceramic cross section at TN 775 °C, tN 240
min, TC 910 °C and tC 120 min. ... 101
Figure 40 – Micrographs showing Knoop hardness indentations with different applied loads of LAS precursor (Neoceram N-0), Nb-MAS precursor, Nb-MAS GC thermal treated at TN
775 °C, tN 360 min, TC 860 °C, tC 90 min and PMAS (Perlucor). ... 102
Figure 41 – Probability of crack initiation of Nb-MAS glass-ceramics and reference materials obtained by Vickers indentations. ... 102 Figure 42 – Images of crack initiation of Nb-MAS glass-ceramics and PMAS. ... 103 Figure 43 – Nucleating agent effect on total forward transmission of MAS. Additionally, MAS’ TFT is compare with some reference materials... 104 Figure 44 – SEM micrographs showing Mo-MAS GC (1.0 mol%) when Mo-MAS precursor glass were melted in air (a). A dominant surface crystallization mechanism was noticed. In contrast, when Mo-MAS precursor glass (0.2 mol%) was melted in a reducing atmosphere (b), it seems that simultaneous surface and volume crystallization mechanisms are induced. ... 105 Figure 45 – Influence of crystallization temperature and time in ILT of Nb-MAS and Ti+Zr-MAS ceramics. An arrow shows the tendency in which ILT increase. Nb-Ti+Zr-MAS glass-ceramics have ILT >70% for TC < 890 °C, in contrast, Ti+Zr-MAS glass-glass-ceramics keep same ILT up to 980 °C, indicating a different kinetic of crystallization. ... 106 Figure 46 – Overlaid ILT, Knoop hardness and surface roughness results of Nb-MAS GCs. ... 108
INDEX OF TABLES
Table 1 – Filter-like methodology proposed to design armor systems... 43
Table 2 – Impurities content in a powder synthesized by thermal decomposition of alum salts (Baikowski S30CR) and by flame spray pyrolysis (Nanocerox) MgAl2O4 powder. ... 49
Table 3 – Magnesium aluminate spinel commercial references. ... 53
Table 4 – Potential scalable PMAS manufacture parameters... 60
Table 5 – Representative examples of transparent and/or tough MAS GC... 64
Table 6 – Parameters for the Buckingham potential. ... 71
Table 7 – Characteristics of the specimens. ... 73
Table 8 – Influence of size and simulation time on the results. ... 73
Table 9 – Nominal glass composition of the materials analyzed in this work. ... 76
Table 10 – Summary of the thermal analysis performed to determine range of interest of the TN and TC to achieve transparent MAS glass-ceramics... 92
LIST OF ABBREVIATIONS ADF – Angle Distribution Function
ALON – ALuminum OxyNitride AGS – Average Grain Size AP – Armor Piercing CIP – Cold Isostatic Press CN – Coordination Number DOE – Design Of Experiment DOP – Depth Of Penetration GC – Glass-Ceramic
HP – Hot Press
HIP – Hot Isostatic Press
HSIR – High-Speed Impact Resistance ILT – In-Line Transmission
IR – Infra-Red
LAMMPS – Large-scale Atomic/Molecular Massively Parallel Simulator LAS – Lithium-Aluminum-Silicate
MAS – Magnesium-Aluminum-Silicate MD – Molecular Dynamics
Mo-MAS - Magnesium-Aluminum-Silicate glass-ceramics nucleated with MoO3
Nb-MAS – Magnesium-Aluminum-Silicate glass-ceramics nucleated with Nb2O5
NPT – Number of particles, N; the pressure, P; temperature, T; are constant NVE – Number of particles, N; the volume, V; energy, E; are constant NVT – Number of particles, N; the volume, V; temperature, T; are constant PDF – Pair Distribution Function
PMAS – Polycrystalline Magnesium Aluminate – Spinel SPS – Spark Plasma Sintering
TAS – Transparent Armor System TGC – Transparent Glass-Ceramics
Ti+Zr-MAS - Magnesium-Aluminum-Silicate glass-ceramics nucleated with TiO2+ZrO2
TABLE OF CONTENTS
1 INTRODUCION ... 21
1.1 GENERAL OBJECTIVE ... 24
1.2 SPECIFIC OBJECTIVES ... 24
2 LITERATURE REVIEW ... 25
2.1 CHARACTERISTICS AND TESTING OF STRIKE FACE MATERIALS ... 27
2.1.1 Optical properties ... 27
2.1.2 Elastic-plastic indentation response ... 30
2.1.3 High-speed single-impact tests... 34
2.1.4 High-speed multi-impact tests ... 40
2.1.5 Design methodology ... 43
2.2 MANUFACTURING TECHNOLOGIES OF STRIKE FACE MATERIALS ... 44
2.2.1 Polycrystalline magnesium aluminate spinel (PMAS) ... 45
2.2.1.1 Raw materials ... 46
2.2.1.1.1 Stoichiometry ... 46
2.2.1.1.2 Impurities ... 48
2.2.1.1.3 Additives ... 50
2.2.1.1.4 Particle size, agglomeration and de-agglomeration ... 51
2.2.1.2 Shaping techniques, homogeneity and green body density ... 52
2.2.1.2.1 Cold isostatic pressing ... 53
2.2.1.2.2 Gelcasting ... 55
2.2.1.3 Thermal processes ... 57
2.2.1.3.1 Combined sintering methods (PS/HIP, HP/HIP) ... 57
2.2.1.3.2 Sintering in conventional furnaces ... 59
2.2.1.3.3 Spark plasma sintering (SPS) ... 59
2.2.1.4 Diamond grinding and polishing ... 61
2.2.2.1 Raw materials ... 63
2.2.2.2 Melting ... 63
2.2.2.3 Thermal treatments ... 65
2.2.2.4 Diamond grinding and polishing ... 67
3 MATERIALS AND METHODS ... 69
3.1 MODELING BY MOLECULAR DYNAMICS ... 70
3.1.1 Interatomic interactions ... 70
3.1.2 Definition of molecular systems ... 71
3.1.3 Structural characterization ... 74 3.1.4 Strain-stress simulation ... 74 3.2 EXPERIMENTAL APPROACH ... 75 3.2.1 Materials ... 75 3.2.2 Processing ... 75 3.2.3 Characterization ... 76
4 RESULTS AND DISCUSSION ... 79
4.1 MOLECULAR DYNAMICS ... 79
4.1.1 Structural analysis ... 80
4.1.2 Strain-stress simulation ... 84
4.1.3 Structural damage under uniaxial strain ... 87
4.2 EXPERIMENTAL RESULTS ... 91
4.2.1 Thermal analysis and crystallization kinetics ... 91
4.2.2 Structural and microstructural characterization ... 95
4.2.3 Mechanical behavior ... 98
4.2.4 Optical characterization ... 103
4.2.5 Summary of optical and mechanical properties ... 107
5 CONCLUSIONS AND PERSPECTIVES ... 109
1 INTRODUCION
Right to life is a fundamental human right that obliges States to protect human life. Nowadays, we are living in the most peaceful era of the history due to a majority of democratic elected governments. Even though, there are still conflicts in our planet, which impair the ideal of a humanity living in peace. Arms and armor have been a vital part of virtually all cultures for thousands of years. Throughout time, the best armor and weapons have represented the highest technical capabilities of the society and period in which they were made. Thus, transparent armor designers have been evaluating novel materials such as transparent ceramics and glass-ceramics as complement of conventional glasses.
Transparent ceramics have been investigated as strike face materials for transparent armor systems (KRELL; STRASSBURGER, 2014a), (SALEM, 2013). However, their cost and performance trade-off is still not ideal, which keep them far from massive field use (PINCKNEY et al., 2011). On the other hand, the conventional lower-cost soda-lime-silicate and borosilicate glasses are inefficient for some threats. In this thesis, we intend to simplify the landscape of transparent armor materials and manufacture. Here, guidelines for materials development are discussed in the form of a filter-like methodology related with the material properties assessment, which is organized from the straightforward, economic evaluation to the most complex, expensive analysis. After that, the materials outlook is reviewed with special attention to Polycrystalline Magnesium Aluminate Spinel (PMAS) and Magnesium-Aluminum-Silicate (MAS). For either PMAS or MAS, the manufacture state of the technique is also reviewed to establish their scale-up potential.
Molecular Dynamics (MD) simulations could aid the development of aluminosilicate glasses with properties tailored to the specific application requirements and, consequently, lead to an improvement in the next-generation of transparent armor systems (TAS). Molecular dynamics modeling has become relevant in the study of binary Al2O3-SiO2 glass systems
(WINKLER et al., 2004). Silicate glasses have been modeled by MD using different potential models, which play a crucial role in the final structure and properties obtained (DU; CORMACK, 2004).
In this thesis, aluminosilicate glasses, xAl2O3·(1-x)SiO2 with variable molar ratio 0 ≤
x ≤ 0.6, were initially modeled by molecular dynamics (MD) in order to investigate the structural characteristics such as: coordination number and pair-angle distribution functions responsible for their mechanical performance. Here, MD simulations models were setup by
replacing SiO2 by Al2O3 to create aluminosilicate systems with different compositions. Thus,
Al atoms were randomly placed at Si positions and O atoms randomly removed to keep the stoichiometry of the system. The short- and intermediate-range structural arrangements of each molecular system were studied by mean of the pair distribution function (PDF), angle distribution function (ADF) and coordination number (CN).
Mechanical properties of binary Al2O3-SiO2 glass systems seem to be
counter-intuitive, because hardness and densification, i.e., crack resistance (KATO et al., 2010) are not achievable simultaneously in oxide glasses; however, these systems can attain a balance between supposedly mutually excluding characteristics (ROSALES-SOSA et al., 2016). Open-structure glasses have enough free space to dissipate mechanical stress by densification; hence, less-packed glasses have been suggested to be used behind glass-ceramics’ strike-face in TAS (PINCKNEY et al., 2011). On the other hand, glass-ceramics have been suggested as suitable strike-face materials due to their high hardness (up to 13.5 GPa) and feasibility (BENITEZ et al., 2017), (PINCKNEY; ZHANG, 2008), (DITTMER; RÜSSEL, 2012). Perhaps a binary Al2O3-SiO2 glass systems with a balance between crack resistance and
hardness, like that developed by Rosales-Sosa et al. (ROSALES-SOSA et al., 2016), could be an efficient solution for TAS, instead of an anomalous material behind the strike face (PINCKNEY et al., 2011) and a glass-ceramic for strike face (PINCKNEY; ZHANG, 2008). In this thesis, tension tests were also simulated in MD to study the mechanisms of structural damage at nanoscale. The mechanical properties estimated using MD are linked to macroscale properties influencing TAS performance.
Low-cost soda-lime-silicate and borosilicate glasses are the traditional brittle materials used in transparent armor systems (TAS). However, those materials are mass-inefficient for some threats (GRUJICIC; BELL; PANDURANGAN, 2012), (BENITEZ et al., 2017). Magnesium-aluminum-silicate (MAS) glass-ceramics nucleated with Nb2O5
(Nb-MAS), could be a more suitable solution than commercial transparent ceramics (KRELL; STRASSBURGER, 2014b), (CERAMTEC, 2016) and glass-ceramics (WEINHOLD, 2013), (PINCKNEY; ZHANG; CLINE, 2011). This is due to the synergy of technical and economic aspects, such as transparency and impact stability, as well as market feasibility, including affordability and scaling up (BENITEZ et al., 2017).
We suggest a magnesium-aluminum-silicate transparent glass-ceramic nucleated with Nb2O5 as an alternative strike face solution to improve TAS performance, following the
presented by previous reports. As a first step, a glass-ceramic composition was selected – pursuing a balance between mechanical properties, transparency and feasibility – and its crystalline volume fraction was estimated.
The glass composition 60SiO2·20Al2O3·20MgO (wt.%) was selected due to its
eutectic point between cordierite and enstatite, which ensures a low liquidus temperature (MAEDA; SERA; YASUMORI, 2016). Additionally, MAS glass-ceramic system was chosen instead of the commonly used commercial lithium-aluminum-silicate due to their hardness value may reach up to 13.3 GPa (SANT’ANA GALLO et al., 2017), (DITTMER; RÜSSEL, 2012) and also due to previously-reported remarkable properties when residual glass fraction was lower than 5% (BEALL, 2008). Finally, Nb2O5, MoO3 and TiO2+ZrO2 were selected as
nucleating agents, following Maeda and Yasumori’s work (MAEDA; YASUMORI, 2015), in order to maximize the crystalline volume fraction, while keeping transmittance as high as possible.
The mechanical properties of aluminum-silicate glass-ceramics are mostly affected by the field strength of the network modifier ion, increasing as field strength increments, with magnesium presenting one of the highest values (TIEGEL et al., 2015). As mentioned before, factors affecting the impact stability are still a matter of controversy (KRELL; STRASSBURGER, 2014a). However, it is widely accepted that the TAS performance is influenced by the mode of fracture, backing stiffness and hardness of its components (PINCKNEY; ZHANG, 2008), (HOGAN et al., 2017). Furthermore, TAS designers have been demanding for a screening technique to identify promising materials earlier in the development of next-generation products (HALLAM et al., 2015), (MULLER; GREEN, 2013).
Thus, we apply the TAS design guidelines previously presented (BENITEZ et al., 2017), by which materials should be evaluated in a straightforward way, from less costly analyses to the most complex-expensive ones. We focus our efforts in microstructural characterization, involving prediction of crystallization kinetics (CHAKRABARTI; BISWAS; MOLLA, 2018), Raman, XRD and SEM analysis. Moreover, lab-scale low-strain-rate techniques, such as hardness and crack resistance measurements, may be linked to impact stability. In parallel, we start to build a procedure protocol using computer-assisted design to predict the mechanical response of glass-ceramics intended as strike face materials for transparent armor systems.
1.1 GENERAL OBJECTIVE
The general objective of this thesis is to design a transparent strike face magnesium-aluminum-silicate glass-ceramic for armor application with a compromise between impact resistance and affordability.
1.2 SPECIFIC OBJECTIVES
• Review the state of the technique of brittle transparent materials used in armor systems to identify the material families with potential to enhance the performance of those systems. • Simulate with molecular dynamics the structure and mechanical properties of
aluminosilicate glasses, xAl2O3·(1-x)SiO2 with variable molar ratio of 0 ≤ x ≤ 0.6.
• Evaluate the influence of nucleating agents on crystallization kinetics and mechanical properties of magnesium-aluminum-silicate glass ceramics.
2 LITERATURE REVIEW
Transparent materials are essential for humanity since the reality perception is mainly conceived through the vision sense (HOFFMAN, 2015). Advanced transparent materials have gradually replaced their traditional counterpart, i.e. glasses, in relevant technological applications where conventional materials are infeasible or unsuitable, e.g. armor systems and space shuttle windows. Here we focus on front materials for transparent armor, also known as strike face materials.
Transparent armor, for instance security windows, is one of the current most important technologies for people protection. Commonly, transparent armor consists in a set of several layers composed by laminates of soda-lime or borosilicate glass, thermoplastic hot-melt adhesives and polymeric anti-spall layers. Nowadays, the requirements for those systems are increasing; thus, the traditional glass-based armor is becoming unpractical mainly for weight and thickness constrains. Hence, a multifunctional design concept is being sought for lightweight armor development and for stringent applications as armored vehicles (COMMITTEE ON OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS, 2011). For illustration, Figure 1 shows a multifunctional design of: A) a projectile-eroding-fragmenting layer (strike face); B) energy-absorbing, crack-arresting layers (internal layer); C) a fragmented-armor debris containment layer (spall shield); and D) adhesive plies (adhesive), used to join the system and give multi-hit resistance (GRUJICIC; BELL; PANDURANGAN, 2012).
The strike face is the core layer concerning the bulk-design. Glass-made armor weight and thickness can be reduced around 30 to 60%, when transparent ceramics as ALON, sapphire or polycrystalline magnesium aluminate spinel (PMAS) are used for this functional layer (U.S. DEPARMENT OF DEFENSE, 2012), (STRASSBURGER, 2009).
A ceramic strike face layer was first proposed with sapphire four decades ago (BALLARD, 1978). Recent works define PMAS as a “game-changing technology” for the upcoming armor development (GRUJICIC; BELL; PANDURANGAN, 2012), (WANG et al., 2013b), (GOCHA, 2015). However, sintering a fully dense and transparent PMAS in conventional furnaces is still a technological challenge; consequently, complex equipment (GOCHA, 2015) and/or multistep procedures (BENAMEUR et al., 2011), (RUBAT DU MERAC et al., 2013), (LAROCHE et al., 2008) are involved in the process to achieve both requirements.
Figure 1 - Functional layers on transparent armor concept design.
Alternative materials, such as bulk nanocrystalline glass-ceramics (GC), are easier to process in comparison to PMAS, essentially because of their lower fusion temperatures. Lithium-aluminum-silicate (LAS) (PINCKNEY; ZHANG; CLINE, 2011), (SIEBERS et al., 2012a), (RODRIGUES et al., 2015), lithium disilicate (BUDD; DARRANT, 1995), and recently magnesium-aluminum-silicate (MAS) have been developed as strike face materials for transparent GC (RODRIGUES et al., 2015).
Goldstein and Krell recently published a review on the state of the technique regarding transparent ceramics (GOLDSTEIN; KRELL, 2016). Here we expand the discussion for armor applications and introduce transparent glass-ceramic materials as an alternative solution for strike face layer of security windows.
In this chapter, materials for transparent armor applications are described, classified and discussed. We discuss design guidelines, bringing useful methods to compare options or assess performance for materials development. The approach herewith is organized from the straightforward, economic evaluation to the most complex-expensive analysis. Those guidelines are intended to help users to gain a better understanding of the material´s structure, properties and performance. Moreover, we focus on representative materials such as Polycrystalline Magnesium Aluminate Spinel (PMAS) and Magnesium-Aluminum-Silicate (MAS), and their manufacture, including scale-up potential.
2.1 CHARACTERISTICS AND TESTING OF STRIKE FACE MATERIALS
As previously mentioned, lightweight armor has recently gained attention for intended applications where size, heaviness-mobility and payload capacity are important factors (TALLADAY; TEMPLETON, 2014). Transparent ceramics and glass-ceramics can reduce weight and thickness when introduced in armor design. However, the design process is still ambiguous, generally because of the failure mechanism complexity during high-speed impact (TALLADAY; TEMPLETON, 2014), (WALLEY, 2014), involving superposition of effects, material-geometry, during projectile-armor interactions. A separation of effects is needed to validate a comparative evaluation of high-speed impact tests with different armor configurations (KRELL; STRASSBURGER, 2013).
Based on previous works, the design process can be further systematic, guiding the quality control to comprise the system whole, and not solely the strike face as commonly performed. For example, Krell and Strassburger suggest a hierarchical classification on a paper series, ranking key properties regarding armor impact resistance (KRELL; STRASSBURGER, 2007), (KRELL; STRASSBURGER, 2008), (KRELL; STRASSBURGER, 2013), (KRELL; STRASSBURGER, 2014a).
In general, three characterization approaches ‒ light transmission, elastic-plastic indentation response, single and multi-impact performance ‒ can be used to assess the suitability of ceramics and/or GC materials as strike face materials. Those characterization approaches are discussed in this literature review from the easiest/cheapest to the most complex/expensive to perform.
2.1.1 Optical properties
Transparency or light transmission is the material ability to allow light to pass through it without scattering (KONG et al., 2015). A medium can be considered transparent, if it allows the formation of an undistorted image of a target scene in a sensor (GOLDSTEIN; KRELL, 2016). Depending on light wavelength and nature of the material, the light shed on it can be reflected, refracted, scattered and/or absorbed (GOLDSTEIN; KRELL, 2016), (KONG et al., 2015). Goldstein and Krell categorize scattering and absorption as the most difficult phenomena to avoid/reduce during transparent ceramic fabrication (GOLDSTEIN; KRELL, 2016).
Translucent polycrystalline ceramics have advantages over single crystal ceramics, such as cost-effectiveness and ease of manufacture of pieces. Nevertheless, the presence of grain boundaries and pores in polycrystalline ceramics affects negatively the transparency and represents a challenge for the densification of those materials during sintering (IKESUE; AUNG, 2008).
In Figure 2, a schematic representation of a polycrystalline ceramic microstructure shows the light interaction sites. Initially, the light has an incident intensity (I0); depending on
the surface roughness, a component of diffuse reflection (RD) and diffuse transmission (IDT)
are created. Additionally, an amount of light is reflected at each surface of the material by specular reflection (RT ~13%). The light traveling through the polycrystalline material can
also interact with pores (residual pores scattering), grain boundaries (secondary phase scattering), impurities (inclusion absorption or scattering), and birefringence (double refraction of non-cubic phases).
PMAS is the ceramic with highest light transmission percentage at a portion of the middle IR wavelength range, as shown in Figure 3. This characteristic gives PMAS an advantage against sapphire and ALON where on-board IR sensors are required (RUBAT DU MERAC et al., 2013). Conversely, spinel is difficult to sinter at near-theoretical density required for transparency (>99.99%) (RUBAT DU MERAC et al., 2013). To obtain PMAS enhanced transparency, it is paramount to improve the sintering process to minimize light scattering and absorption in the part of the electromagnetic spectrum of interest, mainly hindering porosity, avoiding undesirable secondary phases at grain boundaries, obtaining isotropic lattice structures (i.e. cubic) and high-quality surface finishing (KONG et al., 2015).
The light transmission sensibility with pore size is remarkably critical: 0.01% of pores in materials with 400 nm grain sizes at λ of 600 nm can reduce the in-line transmission (ILT) around 50% (RUBAT DU MERAC et al., 2013).
Transparency commonly depends on sample thickness, and the independency is possible for cubic lattice structure materials or for materials with average grain size << λ (GOLDSTEIN; KRELL, 2016). Therefore, cubic lattice structure is preferred for transparent ceramic strike face layers, where relative high thicknesses (>4 mm) are necessary to reach suitable impact resistance (KONG et al., 2015).
Figure 2 - Schematic light transmission phenomena in a polycrystalline ceramic.
Glass-ceramics (GCs) with crystal size below 30 nm are alternative transparent materials (PINCKNEY; ZHANG; CLINE, 2011), (RODRIGUES et al., 2015). Commercial lithium-aluminum-silicate GCs (SCHOTT, 2015), (NIPPON ELECTRIC GLASS, 2014) are not transparent at middle-IR range as shown in Figure 3. Thus, available GCs are not suitable for applications where on-board middle-IR sensors are mandatory.
Requirements for transparent GC are also low-light scattering and absorption (BEALL; PINCKNEY, 1999). Low scattering can be obtained when the present crystalline phases and residual glass have closely matched refraction indexes - difference below 0.1 - (BEALL; PINCKNEY, 1999), crystal low birefringence, and crystal sizes smaller than target wavelength of light. For visible light transparency crystal sizes below 30 nm are suggested (PINCKNEY; ZHANG; CLINE, 2011).
Figure 3 - Light transmission of selected ceramics and glass-ceramics.
0 1000 2000 3000 4000 5000 6000 7000 0 20 40 60 80 100 Middle-IR Near-IR VIS Lig ht Tran smis sion ( %) Wavelength (nm) Spinel Sapphire ALON LAS SCHOTT LAS NEG UV 87% Spinel Theoretical Light Transmission Limit
Currently, commercial GCs with improved ILT properties in the range of wavelength between 0.4 and 6 µm, specifically compatible with on-board IR-sensors, remain as a development challenge.
Light transmission, haze and appearance (i.e. color and blackspots) at visible wavelength range are simple to perform as a first selection test of potential strike face materials. In-line transmission and total forward transmission can be measured with a spectrometer or evaluated qualitatively; haze is the difference between them. Optical defects at visible wavelength can be also estimated by naked eye with the appropriate light and color backgrounds (KRELL; HUTZLER; KLIMKE, 2014).
To our knowledge, the minimum light transmission for armor is not specified at standards (NORTH ATLANTIC TREATY ORGANIZATION, 2004), (NORTH ATLANTIC TREATY ORGANIZATION, 2005), but a GC-based armor suggests a minimum of 70% (SIEBERS et al., 2012b). Therefore, we consider that materials with total forward light and in-line transmission less than 80% and haze higher than 5% (KRELL; HUTZLER; KLIMKE, 2014) should be considered unsuitable, bearing in mind that the strike face is just one layer of the armor system.
2.1.2 Elastic-plastic indentation response
Hardness is the material resistance against localized deformation, which in polycrystalline ceramics depends on the grain size (KRELL; STRASSBURGER, 2007), (HOSFORD, 2005). Hardness also depends on bond strength, grain-boundary toughness, density, and subcritical cracking (KRELL; STRASSBURGER, 2007), (HOSFORD, 2005), (KRELL; BALES, 2011). The procedures to perform the tests are standard (ASTM C1326-13, 2003), (ASTM C1327-15, 2003) and recommendations about how the test should be performed on ceramics are available (SWAB, 2004). Static hardness tests are very simple and cost-effective to perform (HOSFORD, 2005).
Krell and Strassburger (KRELL; STRASSBURGER, 2007) proposed a classification according to key properties that affect the armor impact performance, based on previous ceramic wear resistance (KRELL, 1996). In this case, a significant increase in hardness (~HV10 from 16 to 20 GPa) was achieved through the reduction of the grain size of Al2O3
(~3 to 0.4 µm). The small grain size is associated with a high relative density (98.5˗99.8%) and a low frequency of microflaws at grain boundaries.
Krell and Bales (KRELL; BALES, 2011) determined that PMAS Vickers hardness with load applied of 10 N (HV1) is almost constant for average grain size >5 µm. It slightly increased between 5 and 2 µm, and substantially improved in the submicrometer range. Reducing the grain size from 0.8 to 0.3 µm increases the HV1 from 14 to 16 GPa (Figure 4).
The projectile-target interaction is a highly dynamic process, but there is a linear correlation between static and dynamic Vickers hardness of potential strike face materials (Figure 5) (KRELL; STRASSBURGER, 2014a). However, major attention must be given to the interpretation of results of static hardness, because impact performance comes from different stages of the projectile-armor interaction. Thus, different properties need to be considered, and debatable conclusions can be inferred from hardness results (KRELL; BALES, 2011).
Krell and Strassburger (KRELL; STRASSBURGER, 2008), (KRELL; STRASSBURGER, 2013), (KRELL; STRASSBURGER, 2014a) updated their initial hypothesis (KRELL; STRASSBURGER, 2007) for armor high-speed impact resistance, classifying the mode of fragmentation of transparent ceramics as a top priority variable, and dynamic stiffness (dwell phase) and hardness (penetration phase), as second priority variables (Figure 6). The high-speed impact response, as the wear mechanism may present inconsistency of results, where hardness represents a major influence for some impact performance results and becomes unimportant when armor configurations are changed. For instance, for single-impact tests of Al2O3 with an aluminum backing, hardness shows no
influence, but it increases with hardness using a steel backing (KRELL; STRASSBURGER, 2007).
Additionally, Krell and Strassburger (KRELL; STRASSBURGER, 2013),
(KRELL; STRASSBURGER, 2007), (KRELL; STRASSBURGER, 2008), (KRELL; STRASSBURGER, 2014a), (KRELL; BALES, 2011), (KRELL, 1996) suggested that high hardness (HV10 > 14 GPa; 10 kg ~ 100 N) is beneficial for impact performance if a fragment mode condition is reached, corresponding to an inertia force of debris ~1000 N. Nevertheless, hardness is also governed by the relative density. The last tenths of a percent of porosity does not further improve the hardness, but the porosity maybe decreased beyond due to transparency requirements, which demands porosity below 0.01% (KRELL et al., 2010).
Figure 4 - PMAS hardness results scattering and possible Hall-Petch relationship between hardness and nanometric grain size.
Figure 5 - Correlation between static and dynamic hardness.
Source: Krell and Strassburger, (2014)
0.01 0.1 1 10 100 1000 10000 10 15 20 25 30 Sapphire Single Crystal MgOAl2O3 Single Crystal MgO3Al2O3 MgOnAl2O3 n = 2.5 HV 1000 gf33 HV 80 gf42 Sapphire HV 1000 gf19,42 Ma gn esium Alumina te Hard ne ss (G Pa) Grain Size (m) submicrometer n = 2
Figure 6 - Suggested key properties ranking hypothesis affecting the armor impact resistance.
Muller and Green (MULLER; GREEN, 2010), (MULLER; GREEN, 2011), (MULLER; GREEN, 2013) developed two spherical custom-built indentation systems, consisting in depth-sensing indentation techniques, which are useful to determine stress-strain behavior of potential strike face materials from analysis of load-displacement curves. That indentation stress-strain curve analysis represents a more fundamental approach than the traditional hardness testing, describing the elastic-plastic indentation behavior. A linear correlation between the work of indentation and a parameter named “indentation strain energy density” (MULLER; GREEN, 2013) may be a useful procedure to evaluate armor performance.
Based on indentation stress-strain and indentation work, using fine-grained (0.4 µm) and coarse (1.4 µm) PMAS, it is shown that grain-size is barely related with controlling the hardness of PMAS (MULLER; GREEN, 2011), (MULLER; GREEN, 2013). Apparently, for large plate applications such as transparent armor, the overrun cost involved in reducing grain size and improving hardness are worthless from the costs/performance ratio standpoint. Therefore, grain size in the hundreds of µm are often acceptable for PMAS for armor applications (RUBAT DU MERAC et al., 2013).
Commonly, a linear Hall-Petch relationship is expected between hardness and the square root of grain size (MUCHE et al., 2017). Considering this correlation, PMAS with
grain sizes below 20 nm should be harder than sapphire (24 to 28 GPa, Figure 4) (MUCHE et al., 2017). However, Sokol and co-workers have recently demonstrated an inverse Hall-Petch relationship for PMAS with grain sizes below 30 nm (SOKOL et al., 2017), achieving a maximum hardness (HV2) value of 20 GPa and decreasing to 17.6 GPa when the average grain size goes to 17 nm. The main reason of the hardness reduction is the larger volume fraction of grain boundaries which is 60% when the average grain size (AGS) is 7.2 nm and goes to 20% when the AGS is 30 nm (SOKOL et al., 2017).
As previously mentioned, hardness is related with high-speed impact resistance of armor materials (KRELL, 1996), (KRELL; STRASSBURGER, 2013), (KRELL; STRASSBURGER, 2007), (KRELL; STRASSBURGER, 2008), (KRELL; BALES, 2011), (KRELL; STRASSBURGER, 2014a). However, hardest strike face materials have not always reached the highest impact resistance. Several efforts have been done to correlate hardness with high-speed impact resistance of armor (SWAB, 2004), (MULLER; GREEN, 2013), (HALLAM et al., 2015) to reduce the design efforts of a trial and error approach, which demands high number of samples and expensive tests. Some authors suggest a correlation between Knoop and Vickers hardness with the high-speed impact resistance of armor (SWAB, 2004), others believe that a figure of merit derived from round indenter tests could be effective to predict the penetration behavior of armor materials (MULLER; GREEN, 2010), (MULLER; GREEN, 2011), (MULLER; GREEN, 2013).
To the best of our knowledge, a trustworthy correlation between a specific hardness value and the high-speed impact resistance of transparent brittle materials is unavailable. Notwithstanding, we consider that the elastic-plastic indentation approach (MULLER; GREEN, 2010), (MULLER; GREEN, 2011), (MULLER; GREEN, 2013) has the potential to be a helpful screening and quality control method for strike face material selection and, consequently, for reducing the high-speed impact tests required to design armor systems. Unfortunately, a correlation between microstructure, indentation energy and high-speed impact resistance of strike face materials remains a challenge.
2.1.3 High-speed single-impact tests
The single-impact failure mechanisms of ceramic and glass-ceramic strike face materials are complex and not fully understood (MCCAULEY et al., 2013). To predict the High-Speed Impact Resistance (HSIR) of those materials, it is necessary to know the limiting stress as a
function of hydrostatic pressure, strain, strain rate, and temperature in both damaged (multi-impacts) and undamaged (single-impact) states. Nowadays, modeling to predict the impact damage mechanisms of armor is an active research area (LIU et al., 2016), (KUDRYAVTSEV; SAPOZHNIKOV, 2016). Therefore, an analytical-experimental approach may be useful as a practical guide for armor designers in the industry (GRUJICIC; BELL; PANDURANGAN, 2012).
The strike face materials for armor are usually chosen based on bulk mechanical properties (SHOCKEY; SIMONS; CURRAN, 2010). The common insight is that materials with superior values of mechanical properties, such as hardness, strength, and toughness, should have good HSIR as well (KRELL; STRASSBURGER, 2014a), (SHOCKEY; SIMONS; CURRAN, 2010).
As mentioned earlier, some works proposed a hierarchy hypothesis to explain interactions between small arms projectile (<1000 m/s) and armor material during impact tests (Figure 6). Fragmentation mode is considered as a top priority variable, which is mainly governed by microstructural features and dynamic stiffness of the backing. The PMAS and MAS GCs microstructural characteristics are discussed further in the next section.
The dynamic stiffness of the backing refers to the material behind the strike face (i.e. internal and spall shield layers). Only single-impact results using laminate of glass and polycarbonate backing materials are discussed in this thesis, because of the target applications discussed on this manuscript, which consider transparent backing materials closer to the final application.
The relevance of the dynamic stiffness of the backing is determined by the comparison of impact tests when aluminum or glass backing materials are used. The projectile is undamaged by Al2O3 in front of Al (Figure 7a), whereas the same ceramic erodes the
projectile with glass backing configuration (Figure 7b). PMAS shows similar projectile-damage capability than Al2O3.
The single-impact resistance can be determined through the V50 and depth of penetration (DOP) tests. The V50 test is described in the standard MIL-STD-662F (U.S. DEPARTMENT OF DEFENSE, 1997). Additionally, the MIL-STD-662F standard describes a V0 test as the maximum velocity at which a projectile is expected to consistently fail. V0 can be interpreted as a measurement of the ability to stop an impact.
Figure 8 summarizes the set up for a V0 test for armor. The impact resistance results for ceramics and GC, just like the mechanical properties, have a stochastic behavior and
demand statistical approaches for interpretation. Thus, the expression “consistently fail” should be related with a failure probability at V0 velocity.
Figure 7 - Fragments of a projectile after perforation of Al2O3 using (a) aluminum and (b)
glass backing.
a b
Figure 8 - Schematic impact test set up to measure V0 velocity.
The depth of penetration (DOP) is based on the energy conservation principle. It permits to calculate the transferred energy to the strike face material during the projectile impact (HAZELL, 2010). The DOP test uses the same set up seen in Figure 8, but the sample should be replaced by the assembled. shown in Figure 9 (BLESS, 2013). Initially, an impact test is performed on a quasi-infinite substrate (suggested polycarbonate). Subsequently, transparent armor system (TAS) is joined to the substrate and a second impact is performed. The delta depth of penetration (DOP) is related with the energy absorption capability of the strike face, when the other armor layers are kept invariable. Cross-section B-B and details G clarify the assembled and proposed for the DOP test.
The dwell and penetration stages are classified as a second priority. The dwell phase is the first step during projectile-armor interaction, having a duration of less than 10 µs
(KRELL; STRASSBURGER, 2007). During this phase, a high dynamic stiffness, determined by the Young´s modulus, overmatches the impact load, dwelling the projectile and retarding the penetration.
Figure 9 - DOP test set up for armor using polycarbonate as a substrate.
The strike face materials should be capable to erode the projectile maximizing the dwell time and, as indicated previously, the material may fracture promoting high size debris (1-2 mm) (KRELL; BALES, 2011), (KRELL et al., 2013). The inertia of debris may be large enough to promote an abrasion interaction with the projectile (KRELL; STRASSBURGER, 2007). This is the major reason of projectile deceleration during the penetration phase. Figure 10 shows the debris size distribution of a potential strike face material for armor (KRELL et al., 2013).
The single crystal magnesium aluminate spinel is the material with higher amount of big size debris (1-2 mm). PMAS does not present significant differences from an average grain size of 0.5-36 µm. For those samples, an aluminum backing material was used, and similar tests using glass backing material were performed with non-conclusive results (KRELL; STRASSBURGER, 2014a), (KRELL et al., 2013). This could be due to the difficulty to differentiating strike face and internal layer debris (KRELL et al., 2013).
The penetration stage can be assessed by the elastic-plastic indentation response, as previously discussed. It is suggested that high hardness of strike face fragments is beneficial to improve impact resistance if a fragmentation mode condition is achieved (KRELL;
STRASSBURGER, 2007), (KRELL; STRASSBURGER, 2008), (KRELL; STRASSBURGER, 2013), (KRELL; STRASSBURGER, 2014a). During the penetration stage, the strike face ahead of the projectile loses its cohesion strength and is severely fragmented. Subsequently, the strike face fragment ejects from the cavity created by the impact, and it is assumed that the ejected fragments were in contact with the projectile. Thus, an analysis of the fragment size can give information related with the strike face fragmentation mode and dynamics of impact phenomenon (KRELL; STRASSBURGER, 2013).
Figure 10 - Fragment mode of strike face materials after DOP test.
Source: Krell et al., (2013)
Shockey et al. (SHOCKEY; SIMONS; CURRAN, 2010) suggested a complementary explanation of variables influencing the penetration stage of strike face materials along with four ways to enhance their performance by employing:
• Microstructures capable to resist cone and lateral cracking. • Microstructures that are more difficult to comminute.
• Materials with trend to break into fragment geometries that are more resistant to flow. • Formation of more dilatant fragment beds.
0.025-0.063 0.063-0.1 0.1-0.2 0.2-0.5 0.5-1.0 1.0-2.0 0 5 10 15 0 5 10 15 Mass of Ce ra mic Frag ments (g)
Size Interval of Ceramic Fragments (mm)
Spinel Single Crystal (111) Spinel (AGS 0.55 um) Spinel (AGS 1.4 um) Spinel (AGS 13.2 um) Spinel (AGS 36.3 um) Sapphire (0001) Sapphire (1120)
Figure 11a is a summary constructed from single-impact results (STRASSBURGER, 2009), (KRELL; STRASSBURGER, 2007). As previously discussed, the MIL-STD-662F standard (U.S. DEPARTMENT OF DEFENSE, 1997) suggests to vary the projectile velocity in order to find the maximum speed at which a 7.62 mm 51 AP projectile is expected to consistently fail. Initially, a single-impact test is performed at constant projectile velocity (850 ± 15 m/s), varying the armor characteristics: strike face material, strike face thickness and areal density. Therefore, the results should be interpreted as the minimum areal density capable to stop a 7.62 mm 51 AP at 850 m/s, for different strike face materials and thicknesses. Figure 11a shows that an areal density ~63 kg/m2 is enough to stop the impact
using alumina, ALON and spinel; in contrast, sapphire requires 77 kg/m2. Glass and GC
samples require at least areal densities of 130-160 kg/m2.
Figure 11b, constructed from DOP test performed using soda-lime glass backing (KRELL; STRASSBURGER, 2014a), (KRELL et al., 2013), complements the information obtained from the V0 and DOP tests with Al backing material (Figure 11a). It is well-known that impact results obtained with Al backing cannot be extended to DOP with glass backing (KRELL; STRASSBURGER, 2013), (KRELL; STRASSBURGER, 2007), (KRELL; STRASSBURGER, 2008), (KRELL; STRASSBURGER, 2014a). The materials tested show a DOP between 5 and 15 mm for strike face thickness ˃2 mm; however, for thickness below 2 mm, the DOP is remarkably high (>20 mm) and unsuitable for use in armor applications. The DOP of single crystal spinel, PMAS (AGS 0.4 µm to 36 µm) and alumina (AGS 0.6 µm to 22 µm) are comparable. Thus, it seems to be unnecessary to refine the grain size of ceramics strike face materials when a glass backing internal layer is used, which is close to the final application.
Figure 11 - Single-impact resistance of potential strike face materials (a) minimum areal density and strike face required to stop a 7.62 mm 51 AP, steel core projectile and (b) DOP results of PMAS and alumina with glass backing.
2.1.4 High-speed multi-impact tests
The high-speed multi-impact (HSMI) failure mechanisms are more intricate than single-impact (HSSI), as consequence of the fracture pattern of the brittle material, which presents a random behavior. The multi-impact tests can be seen as a multiple DOP; thus, there are no clear relationships between multi-impact failure, stresses and macro, micro, and nanostructural properties of transparent armor materials (MCCAULEY et al., 2013).
Armor manufacturers and consumers establish the value of a product based on two main characteristics, price and the multi-impact resistance, related to areal density, thickness and threat tested. Figure 12 summarizes the armor available products for the STANAG 4569 level 2 (Figure 12a) and level 3 (Figure 12b) separated by material families (ORAN SAFETY GLASS, 2015), (GKN AEROSPACE, 2015), (ISOCLIMA, 2014), (AGP, 2015), (CERAMTEC, 2016), (TECHNOLOGY ASSESSMENT & TRANSFER, 2016), (ARMORLINE CORPORATION, 2013). Additionally, some data were compiled from personal interviews with people involved in the armor market.
The market dynamics, as well as research and development efforts, are motivated by the consumer’s interest in lightweight armor and by cost reductions, when improving impact resistance. It is also desirable to expand the electromagnetic spectrum range in which the armor is transparent to develop new applications. Conversely, manufacturers work towards lightweight armor development to raise its market value.
Figure 12a and 12b show how ceramic-based armor are more cost intensive, lighter, and thinner than glass-ceramic and glass-made armor. Additionally from the graph, it is possible to infer how much weight and thickness reduction is possible when a glass-ceramic strike face layer is used.
The ceramic strike face material may be the target in terms of weight and thickness for novel armor research. Regarding costs, the small bubble size may be the target for further developments.
To reduce costs, the manufacturing steps may be reduced or alternative materials should be sought. In the former approach, one-step sintering methods are being searched to obtain fully dense materials. In the latter, alternative materials have been suggested, such as transparent glass-ceramics with higher impact resistance (>20% single-impact) (KRELL; STRASSBURGER, 2008), (COMMITTEE ON OPPORTUNITIES IN PROTECTION MATERIALS SCIENCE AND TECHNOLOGY FOR FUTURE ARMY APPLICATIONS, 2011), than lithium aluminosilicate, lithium disilicate or MAS currently used for armor (WEINHOLD, 2009).
Figure 12 - Transparent armor materials for STANAG 4569: (a) Level 2, and (b) Level 3.
Figure 12 summarizes current efforts to develop transparent armor with favorable balance between cost (<7 times glass-based armor cost), multi-impact resistance (>30% lightweight and >50% thinner), and light transmission (>80% at visible wavelength).
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