The effect of carbon and boron carbide additions in pressure assisted sintered silicon carbide Henriette Skarpeid Materials Technology Submission date: June 2017 Supervisor: Kjell Wiik, IMA Co-supervisor: Pål Runde, Saint-Gobain Ceramic Materials Mari-Ann Einarsrud, IMA Norwegian University of Science and Technology Department of Materials Science and Engineering Preface The work described in this master thesis has been performed at the Department of Materials Science and Engineering at the Norwegian University of Science and Technology(NTNU)duringthespringof2017. TheworkpresentedinthisMaster thesis is partly built on the project report "Densification of silicon carbide by hot- pressing and the effect of carbon" from the fall 2016, by the same author. This work has been a collaboration between the Norwegian University of Sci- ence and Technology and Saint-Gobain Ceramic Materials AS, and both parties have contributed to the project. Most of the work and experiments have been exe- cutedattheDepartmentofMaterialsScienceatNTNU,exceptpowderproduction and etching of samples, which took place at Saint-Gobain Ceramic Materials AS, Lillesand. During that period numerous people have been involved in this project and providedmewiththeirassistance. Firstofall,Iwouldliketothankmysupervisor, Professor Kjell Wiik for his time, guidance, and support and my co-supervisor Mari-AnnEinarsrud. IwouldalsoliketothankPålRunde,R&DdirectoratSaint- Gobain Ceramic Materials AS, Lillesand for the opportunity to work with silicon carbide in cooperation with the industry. Further, I would like to thank Development Engineer Benoit Watremetz and R&D Engineer Kent Mogstad at Saint-Gobain for much-appreciated knowledge, advice and discussions, which has helped me gain insight into the world of silicon carbide. IwouldalsoliketothankallthetechnicalstaffattheDepartmentofMa- terialsScienceandEngineeringatNTNUforallthesupportwiththeexperimental part. Lastly, I would like to thank all the members of Inorganic Materials and Ceramics Research Group for all the helpful advice I received during the semester. I II Abstract Siliconcarbideisahighlycovalentceramic,soadditiveslikecarbonandboronmust be present in order to obtain a dense material during sintering.1 The focus of this projectistoinvestigatehowtheamountofcarbonandboroncarbideaddedeffects the density, phase composition and mechanical properties of pressure-sintered sil- icon carbide. Three different commercial SiC-powders, Densitec 13H (13 m2g−1), Densitec 15H (15 m2g−1) and Densitec 13HR (13 m2g−1) were tested with differ- ent amounts of carbon, ranging from 0-2.5 wt%. The powders were hot-pressed at 2050°C with 20 MPa pressure for one hour. Densitec 13HR was also tested with boron carbide content from 0.2-1.7 wt%. These powders were spark plasma sintered at 2050 °C with 20 MPa pressure for 10 minutes. All samples were then polished and characterised. Densitec13HobtainedhigherdensityforlowercarbonconcentrationthanDen- sitec 15H, while Densitec 15H had the higher density when the amount of carbon exceed 1.0 wt%. The hardness measured with Vickers micro-indentation showed a similar trend. Both samples achieved densities above 98.5 % with no carbon addition, and a hardness of approximately 2750 HV. In Densitec 13H, the specific surface area is smaller than in Densitec 15H. Therefore, less carbon is consumed duringsintering,andwillhavemorecarbonpresentonthegrainboundaries,which will decrease the density and hardness. Densitec 13H, which contains carbon black, was further compared to Densitec 13HR, with resin as carbon source. Both density and hardness measurements showed very similar values regardless of the amount of carbon. Densitec 13H had more large anisotropic grains, which increased the fracture toughness. The opti- mised boron carbide content based on density and mechanical properties are 0.7 wt%, which is lower than the concentration used today (1.2 wt%). The SPS- samples had considerable grain growth during sintering. The large grains affected the mechanical properties and resulted in high hardness, but low fracture tough- ness. III IV Sammendrag Silisiumkarbideretmegetkovalentkeramiskmaterialeslikattilsetningsstoffersom bor og karbon er nødvendig for å oppnå et tett materiale under sintring. Fokuset i denne oppgaven er å undersøke hvordan ulike mengder av karbon og borkarbid påvirkertettheten,fasesammensetningenogdemekaniskeegenskapenetiltrykksin- tret silisiumkarbid. Tre forskjellige kommersielle SiC-pulvere, Densitec 13H (13 m2g−1), Densitec 15H (15 m2g−1) og Densitec 13HR (13 m2g−1), ble testet med forskjellig mengde karbon, fra 0-2,5 vekt%. Pulveret ble varmpresset ved 2050 °C med20MPatrykkientime. Densitec13HRbleogsåtestetmedborkarbidinnhold fra0,2-1,7vekt%. Dissepulvereneblesparkplasmasintretved2050°Cog20MPa trykk i 10 minutter. Alle prøvene ble deretter polert og karakterisert. Densitec 13H oppnådde høyere tetthet for lavere karbonkonsentrasjon, mens Densitec 15H hadde høyere tetthet da mengden karbon oversteg 1,0 vekt%. Hard- heten, som ble målt med Vickers mikrohardhetsmåler, viste en lignende trend. Beggeprøvenehaddetettheterover98,5%utenkarbontilsetningogenhardhetpå ca. 2750 HV. I Densitec 13H er det spesifikke overflatearealet mindre enn i Den- sitec15H.Derforblirdetforbruktmindrekarbonundersintring,slikatdetermer karbon til stede på korngrensene, noe som vil redusere tettheten og de mekaniske egenskapene. Densitec13H,sominneholdercarbonblack,blevideresammenlignetmedDen- sitec 13HR, med resin som karbonkilde. Både tetthet og hardhetsmålinger viste svært like verdier uavhengig av mengden karbon. Densitec 13H hadde mange flere anisotropiske korn, noe som økte bruddseigheten. Det optimaliserte borkar- bidinnhold basert på tetthet og mekaniske egenskaper er 0,7 vekt%, som er lavere enn konsentrasjonen som brukes i dag (1,2 vekt%). SPS-prøvene hadde betydelig kornvekst etter sintring. De store kornene påvirket de mekaniske egenskapene og resulterte i høy hardhet, men lav bruddseighet. V VI Table of Contents Preface I Abstract III Sammendrag V 1 Introduction 1 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Aim of work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 Literature Review 5 2.1 Silicon carbide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.1 The structure of silicon carbide . . . . . . . . . . . . . . . . . 6 2.1.2 Properties of the most common SiC-structures . . . . . . . . 7 2.1.3 Production of silicon carbide . . . . . . . . . . . . . . . . . . 9 2.2 Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3 Sintering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.4 Sintering of silicon carbide . . . . . . . . . . . . . . . . . . . . . . . . 17 2.4.1 The effect of carbon . . . . . . . . . . . . . . . . . . . . . . . 18 2.4.2 The effect of boron . . . . . . . . . . . . . . . . . . . . . . . . 23 3 Experimental 27 3.1 Powders and Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.2 Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.2.1 Spray-drying . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.2.2 Hot-press . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 VII TABLE OF CONTENTS 3.2.3 Spark Plasma Sintering (SPS). . . . . . . . . . . . . . . . . . 31 3.2.4 Surface polishing and etching . . . . . . . . . . . . . . . . . . 33 3.2.5 Phase analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.2.6 Density measurement . . . . . . . . . . . . . . . . . . . . . . 34 3.2.7 Mechanical properties . . . . . . . . . . . . . . . . . . . . . . 34 3.2.8 Scanning Electron Microscopy. . . . . . . . . . . . . . . . . . 35 4 Results 37 4.1 Microstructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 4.2 Density measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.3 Phase Compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4.4 Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.4.1 Hardness measurements . . . . . . . . . . . . . . . . . . . . . 52 4.4.2 Fracture toughness . . . . . . . . . . . . . . . . . . . . . . . . 54 4.4.3 Failure analysis . . . . . . . . . . . . . . . . . . . . . . . . . 56 5 Discussion 61 5.1 Microstructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 5.2 Density measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 63 5.2.1 Phase composition . . . . . . . . . . . . . . . . . . . . . . . . 65 5.3 Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . 68 5.3.1 Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 5.3.2 Fracture toughness . . . . . . . . . . . . . . . . . . . . . . . . 70 5.3.3 Failure analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 71 6 Conclusion 73 7 Further work 75 A Powder 85 A.1 Product specification for powder and spray-dried SiC . . . . . . . . . 85 B Experimental tests 89 B.1 Denisty measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 89 B.2 Phase composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 B.3 Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . 92 VIII