DEVELOPMENT OF ELECTRICAL DISCHARGE MACHINABLE AND WEAR-RESISTANT SPARK PLASMA SINTERED SiC BASED COMPOSITES

Abstract

Owing to the unique set of properties like high hardness, strength, chemical stability, low thermal expansion, and low density, silicon carbide (SiC) ceramics are preferred for wear-resistant components such as cutting tools, dies, nozzles, brake pads, nose caps, and leading edges of space vehicles, etc. However, applicability for SiC ceramics is limited because of the difficulty in sintering and shaping. The poor sintering characteristics of these covalent bonded and high melting point materials can be improved by sintering in optimized conditions with appropriate additives and reinforcement. It is to be noted that improved sinterability leads to refined microstructures and enhanced mechanical properties, which result into superior tribological performance. However, machining SiC ceramics into complex shapes is difficult due to their high hardness and brittleness. Therefore, it is understood that improved hardness is needed for superior tribological performance, but high hardness is detrimental for machining. The machining of hard and brittle SiC ceramic workpiece is possible if direct contact with the machining tool is avoided. In this regard, a non-conventional machining technique namely electrical discharge machining (EDM) is useful where the material removal occurs by electrical discharge. However, electrical discharge is possible only when the material exhibits minimum electrical conductivity of 10-2 (Ω·cm)-1. Being a poor/semiconductor, SiC ceramics are machinable by EDM only when reinforced with a conductive second phase. Metallic additives or reinforcements are not recommended because of the possible oxidation and degradation of strength and hardness. Among ceramic reinforcement materials, borides (TiB2, HfB2, etc.) and nitrides (ZrN, TiN, etc.) with high electrical conductivity, high melting point temperatures and high hardness are recommended for development of electrical discharge machinable SiC components for high temperature tribological applications. TiB2 with high strength, modulus and good electrical conductivity is suitable reinforcement to improve mechanical properties and electrical conductivity of SiC ceramics, and ZrN with high melting temperature and high electrical conductivity is suitable to improve electrical discharge machinability and high temperature wear resistance of SiC ceramics. However, literature on EDM and tribological performance of SiC-TiB2/ZrN composites is limited. This can be probably due to difficulty in sintering SiC-boride/nitride composites as higher temperatures and longer holding periods are needed because of poor diffusion characteristics of both matrix and reinforcement. Spark plasma sintering (SPS) is an advanced sintering technique with high rates of heating which can reduce sintering temperatures and shorten sintering time. Further, large control over sintering cycles in SPS helps in obtaining microstructures with homogenous and i connected network of second phase that can improve conductivity and subsequent EDM ability. In this context, an attempt is made to develop new electrical discharge machinable and wear resistant non-oxide composites of SiC with boride, nitride or boride-nitride reinforcement by liquid phase spark plasma sintering in the present thesis work. A total of three SiC-based composite systems: SiC-TiB2, SiC-ZrN and SiC-(ZrN-TiB2) are developed. AlN-Y2O3 additive system was used to facilitate liquid phase sintering. For each composite system, the effect of reinforcement type and content on densification, microstructure, mechanical properties, EDM and wear resistance is primarily discussed. The mechanisms of material removal in sliding wear contacts are elucidated. Firstly, SiC composites with (0 to 30vol%) TiB2 content and 10vol% AlN-Y2O3 additives were prepared by spark plasma sintering at the lowest temperature of 1800 ℃ for 10 min. Densification analysis emphasized improved density with increase in TiB2 content due to beneficial effect of nascent TiO2 presence in decreasing the melting point of liquid phase additives. The phase analysis and microstructure confirm restricted β→α phase transformation in the SiC matrix due to reduced sintering temperature and time. SiC-20vol% TiB2 composite exhibited a better combination of hardness (23.7 GPa) and fracture toughness (5.5 MPa.m0.5). Crack deflection and residual stress development are responsible for improved fracture toughness. For SiC-30vol% TiB2 composites, significant amount of conductive reinforcement, additive composition, and sintering in the nitrogen atmosphere resulted in high electrical conductivity (2.72 × 103 (Ω.cm)-1). The electrical discharge machining of sintered composites illustrates the potential of the sintered SiC-TiB2 composites in extending the application regime of conventional SiC-based ceramics. To assess tribological potential of sintered SiC-TiB2 composites, dry unlubricated sliding wear tests (unidirectional and reciprocating) against SiC ball were conducted at 5, 10 or 15N load in ambient conditions, and at 15N load and 600 ℃ (reciprocating wear). The friction and wear results are reported as a function of TiB2 content. The steady-state coefficient of friction increased with increase in TiB2 content, whereas wear rate decreased with increase in TiB2 addition. At room temperature, abrasion, fracture and grain pull-outs are commonly observed on worn surfaces. Wear mechanisms changed from mechanical wear at room temperature to tribo oxidation at 600 °C. The formation of tribo oxide layer and load bearing roller debris formation resulted in lower wear at 600 °C compared to room temperature. At 600 ℃, the coefficient of friction and wear rate decreased. Though the dominant mechanical wear is observed at room ii temperature sliding, SiC-TiB2 composites exhibited lower wear rate in unidirectional sliding, compared to reciprocating sliding. This result is supported by the contribution of debris (different morphology) with respect to the sliding test configuration. In the second part, dense SiC-(10 to 30vol%) ZrN composites were developed using spark plasma sintering with 10vol% AlN-Y2O3 additive at 1800 °C for 5 min in nitrogen atmosphere. The ZrN addition significantly contributed towards enhancing densification during sintering of SiC composites by reducing melting point and viscosity of liquid phase with ZrO2 presence. While phase and microstructural analysis confirm major β-SiC and ZrN phase after sintering, hardness decreased due to relatively less hard ZrN phase addition and fracture toughness increased due to residual stress development and crack deflection at ZrN phase. Electrical conductivity increased with conductive ZrN addition. SiC-ZrN composites were successfully cut by using wire-EDM. The tribological potential of novel electrical discharge machinable SiC-ZrN composites at various conditions of reciprocating sliding wear contacts was investigated. Results show that friction is less affected by reinforcement content at room temperature (25 ℃), but significantly changed at high temperature (600 ℃). At least one order of lower wear rate obtained with an increase in temperature from 25 ℃ to 600 ℃. The formation of protective (ZrO2.SiO2) rich tribo layer is responsible for superior sliding wear resistance at 600 ℃. In the third part, dense SiC composites with (10 to 30vol%) hybrid (ZrN-TiB2) reinforcement were fabricated by liquid phase spark plasma sintering. The effect of co-addition of ZrN and TiB2 on densification, mechanical properties, and electrical conductivity of SiC ceramics was investigated. In addition to the retardation of β→α phase transformation in SiC, solid solutioning of reinforcement phases and in situ hBN formation are observed in sintered composites. The fracture toughness ranged from 4.1 to 4.6 MPa.m0.5, hardness from 19.5 to 20.3 GPa, and materials removal rate in wire EDM from 7.19 to 8.54 mm2/min with change in hybrid reinforcement content. In reciprocating sliding wear study, almost one order of magnitude reduction in wear rate observed for SiC-ZrN-TiB2 composites with increase in sliding test temperature from 25 ℃ to 600 ℃. The presence of soft liquid phase precipitates and in situ hBN phase resulted in severe fracture at room temperature, while the formation of a protective tribo oxide layer is attributed to lower wear rate at 600 °C compared to room temperature. Overall, the present research study demonstrated the potential of newly developed spark plasma sintered SiC composites with TiB2 and/or ZrN reinforcement for superior performance in electrical discharge machining and sliding wear conditions. The findings from the present study iii are important for the applications in structural, aerospace and defense fields where complex shaped wear resistant SiC ceramic components are needed. SiC composites with 20vol% reinforcement content exhibited better combination of mechanical properties, whereas SiC composites with 20 or 30vol% reinforcement content exhibited superior machining and wear performance compared to monolithic SiC.