TRIBOLOGICAL BEHAVIOUR OF SiC-WC COMPOSITES

Abstract

Silicon carbide (SiC) ceramics are considered as suitable materials for various structural applications such as nozzles, heat exchanger tubes, mechanical seals, bearings, cutting tools or cylinder liners because of their superior properties of high hardness, high temperature strength and excellent resistance to wear and corrosion. Accordingly, extensive research has been carried out towards estimating the tribological potential of these ceramics in various wear conditions. One promising approach for improving wear performance is to combine the properties of different materials. Examples are SiC–TiC composites for improved fracture toughness and Si3N4–SiC composites for improved strength. A new approach that has received much less attention is the incorporation of WC in SiC ceramics. As SiC has higher hardness and oxidation resistance, and WC has better strength and fracture toughness, superior wear resistance is expected for SiC-WC composites. In order to thoroughly assess the potential for different tribological applications such as bearings for the liquid rocket engine turbopumps, bearings for rotary shafts, nozzles, heat exchangers, transportation medium for hot abrasive materials, turbine blades, cylinder liners, and cutting tools etc., a systematic investigation on the behaviour of hot pressed SiC-WC composites in continuous/reciprocated sliding wear and solid particle erosive wear conditions is made for the first time in the present work. The study particularly emphasizes the effect of WC content in SiC ceramics and wear test parameters on tribological behaviour of SiC-WC composites, and provides understanding on material degradation mechanisms. First chapter contains a brief introduction of SiC ceramics and major objectives of present thesis. The second chapter gives a comprehensive literature review on sliding and erosion behaviour of SiC ceramics and SiC based composites. Particularly, effect of microstructure and mechanical properties on tribological behaviour of SiC based ceramics is highlighted. The third chapter deals with the details of experimental procedure carried out in line with scope of the work. Details of composition and method of preparation of investigated materials is reported and this is followed by characterization of microstructures and evaluation of mechanical properties of SiC-WC composites. Details of experimental techniques involved in estimating tribological behaviour of the composites in sliding wear, reciprocating sliding wear and erosion wear conditions are explained. The analytical methods to understand the surface ii of unworn or worn composites are also explained. Further, techniques used for surface and subsurface characteristics of worn surfaces are described. The present work mainly includes four major parts. The first study dealt with continuous sliding wear behaviour of SiC-WC composites. In this part of the study, hot pressed SiC ceramics was subjected to dry sliding wear at 5 N, 10 N and 20 N load against SiC (Hardness: 28 GPa), WC-Co (Hardness: 14 GPa) or steel (Hardness: 7 GPa) ball. Experimental results indicated highest friction against WC-Co ball and highest wear against SiC ball at a given load. Mechanical fracture and abrasion are observed as major mechanisms for material loss against any ball. Extensive fracture of worn surfaces observed for SiC ceramics against SiC ball as compared WC-Co or steel ball at 20 N load. Considering maximum wear at 20 N load against any ball, SiC-WC composites were subjected to sliding wear against different counterbody at 20 N load. Friction decreased with WC content against SiC or WC-Co ball while it increased against steel ball. Generation of hard iron tungsten oxide (FeWO4) debris in ceramics during sliding against steel is believed to cause high friction. The increased fracture toughness of SiC ceramics with WC content caused reduction in extent of fracture during sliding against any ball and led to reduced wear. Significant change in major wear mechanism of SiC-WC composites is observed with change in counterbody. Against SiC ball, SiC-WC composites showed mechanical fracture as dominant wear mechanism, while worn surfaces of composites revealed tribochemistry with increased WC content against WC-Co or steel ball. Frictional behaviour of composites was independent of ball hardness, while wear influenced by the hardness of counterbody and fracture toughness of SiC-WC composites. The second part of study was aimed to understand the tribological behaviour of SiC-WC composites in reciprocated sliding wear conditions. Based on the highest wear obtained against SiC ball in first part of the study, the investigated composites were subjected to unlubricated reciprocating sliding wear against SiC balls at 6 N, 9 N or 19 N load at room temperature and 500oC. The friction decreased with load and WC content at room temperature. SiC-WC composites exhibited maximum wear resistance with 50 wt% WC in room temperature and 30 wt% WC at high temperature and 19 N load. Effect of high humidity (55±5% RH) (compare to (40 ±10% RH) in continuous sliding study) is observed to dominate responsible mechanisms of material removal in SiC-WC composites. Worn surface analysis indicated tribochemistry and microfracture as dominant wear mechanisms for sliding in ambient conditions, whereas microfracture dominated at 500oC. Wear results obtained at high temperature are in consistent with the lateral fracture model for wear volume estimation. Friction and wear results in reciprocated iii sliding conditions advocated the effect of fine grain size and improved mechanical properties of SiC-WC composites. In the third part of study, the SiC-WC composites were subjected to SiC particle erosion wear at high temperature (800oC). In particular, influences of WC particles and angle of impingement (30o, 60o or 90o) of erodent on erosion performance were evaluated. The erosion rate of the composites increased with increased impingement angle from 30o to 90o, and decreased up to 30 wt% WC content. SiC ceramics prepared with 30 wt% WC exhibited highest wear resistance at a given angle of impingement of erodent. Maximum erosion wear rates were obtained for SiC-50 wt% WC composites at normal impact. Worn surfaces revealed grain fracture and pull-out as major mechanisms of material removal for the composites in selected high temperature erosion conditions. Reduction in grain fracture and pull-out observed with decrease in angle of impingement. Owing to the highest hardness the SiC-30 wt% WC composites showed lowest erosion loss. Weak bonding with agglomerated WC particles and severe fracture of SiC grains at normal impact led to a large amount of material loss for SiC-50 wt% WC composites. It is observed from the experimental results that the performance of the composites in wear conditions is largely influenced by the dominant wear mechanisms of material removal. Therefore subsurface of worn composites were systematically studied in the last part of the thesis to elucidate source of material loss. Subsurface analysis under the worn region after dry sliding wear of SiC-WC composite against SiC ball was studied to assess the source of material removal mechanism. Focused ion beam (FIB) cross sectioning on worn surface of SiC and SiC-50wt%WC composite is done to investigate the damage beneath the worn surface. SiC ceramics showed significant damage of ~ 1 μm thickness beneath the worn region, while damage beneath the worn region of SiC-50 wt% WC composites limited to ~ 300 nm. Beneath the damage region, cracks were initiated as microcracks and extended up to ~2 μm downwards the worn surface for SiC ceramics; eventually leading to material removal. TEM analysis of damaged zone of SiC-50 wt% WC composite demonstrated a network of stress induced dislocations and twins in SiC grains. Considerable restriction in the crack propagation by deflection/bridging is observed by WC particles in the SiC-50 wt% WC composites. Results obtained from the series of experiments in the present research truly illustrate the significant role of composition (microstructure), mechanical properties and experimental conditions on the tribological performance of SiC-WC composites. A variation in WC content leads to a considerable difference in iv microstructure and mechanical properties, and hence affected the tribological behaviour of composites. Wear test parameters like load, counterbody, temperature and angle of impingement additionally influence friction and wear characteristics of SiC-WC composites by changing the dominant wear mechanisms. In the backdrop of the present experimental research, it can be concluded that addition of WC is recommended for applications in wear conditions. SiC-50 wt% WC composites are not recommended for use in sliding wear conditions at elevated temperatures. Overall 30 wt% WC addition would be beneficial in any wear conditions when compared to monolithic SiC ceramic.