SiC Whisker-reinforced Alumina Research Articles

Wear Mechanisms and Notch Formation of Whisker-Reinforced Alumina and Sialon Ceramic Tools during High-Speed Turning of Inconel 718.

Nickel-based alloys, referred to as the most difficult-to-cut materials, pose a great challenge to cutting tool materials due to their excellent high-temperature properties. Ceramic tools have the potential to improve the machinability of these alloys with the advance of toughening mechanisms. In this work, the wear mechanisms of SiC whisker-reinforced alumina and Sialon when high-speed turning Inconel 718 alloy under dry cutting condition were investigated. The results showed that the wear process of Al2O3-SiCw WG300 was dominated by the notch wear, while the flank wear characterized by ridges and grooves perpendicular to the cutting edge was the main wear mode for Sialon SX9. A Ti−enriched belt was found at the boundary of the wear band for both ceramic tools. The SEM inspection and EDS analysis for this belt suggested the trace of diffusion between the workpiece material and tool matrix. As for the notch formation, the periodically adhesive action of the workpiece material at the depth-of-cut line combined with the thermal shock resistance of ceramic tools were considered to account for its formation. In addition, the oxidation of the workpiece material at the depth-of-cut line played a positive role in reducing the adhesive affinity and consequent notch wear.

Tool wear and surface roughness analysis in milling with ceramic tools of Waspaloy: a comparison of machining performance with different cooling methods

Ceramic cutting tools are widely used particularly in high-speed machining of difficult-to-machine materials. However, using cutting fluid with these ceramic tools significantly reduces tool life. Therefore, the inclusion of a cooling/lubrication method into the process to improve the machining performance of ceramic tools will make machining efficiency much more effective. The aim of this study is to analyze the effect of cutting parameters and cooling/lubricating conditions on tool wear and surface roughness in the milling of nickel-based Waspaloy with ceramic tools. The cutting tools selected for the study were Ti[C, N]-mixed alumina inserts (CC650), SiC whisker-reinforced alumina inserts (CC670) and alumina and SiAlON ceramic inserts (CC6060). The machining parameters comprised three different cooling/lubricating methods (dry, wet and MQL), three different cutting speeds (500, 600 and 700 m/min) and three different feed rates (0.02, 0.04 and 0.06 mm/rev). Analysis of variance was used to determine the effects of the machining parameters on tool wear and surface roughness. In addition, a regression analysis was conducted to identify the relationship between the dependent and independent variables. According to the experimental results, the minimum quantity lubrication method was identified as the best cooling method for minimum tool wear and surface roughness. In terms of ceramic grades, the SiAlON inserts provided better results in all experimental trials. The dominant wear types observed in all cutting tools were flank wear and notch wear.

Evaluation of machinability of hardened and cryo-treated AISI H13 hot work tool steel with ceramic inserts

The positive effects of deep cryogenic treatment on the wear resistance of cutting tools and workpiece material are well known; however, no information has been reported about the effect on the machinability of cryo-treated tool steel in hard turning. In order to investigate the effects of cryogenic treatment on the machinability of hardened and cryo-treated tool steel, a number of investigations were performed on the hard turning of cryo-treated AISI H13 hot-work tool steel with two ceramic inserts under both dry and wet cutting conditions. Three categories of the hot-work tool steel were turned in the machinability studies: conventional heat treated (CHT), cryo-treated (CT) and cryo-treated and tempered (CTT). Experimental results showed that the lowest wear and surface roughness (Ra) values were obtained in the turning of the CTT samples. Additionally, in terms of main cutting force (Fc), surface roughness (Ra) and tool wear, Ti[C, N]-mixed alumina inserts (CC650) showed a better performance than SiC whisker-reinforced alumina inserts (CC670) under both dry and wet cutting conditions. The use of cutting fluid slightly improved the machinability of the tool steel.

Wear mechanisms of silicon carbide-whisker-reinforced alumina (Al2O3–SiCw) cutting tools when high-speed machining aged Alloy 718

The paper is aimed at the identification and characterization of wear mechanisms of SiC whisker-reinforced alumina when turning aged Alloy 718 under different cutting conditions and when machining dry and with coolant. Secondary and backscatter electron microscopy accompanied by focus ion beam milling and EDX techniques were used for analysis of worn-out tools. Notch wear on the major cutting edge was found to consist of two notches: depth-of-cut notch and secondary notch located outside the chip area. The last was found to be governed by adhesion and attrition associated with adverse chip flow conditions. Formation of a minor notch was related to attrition by the defects found on the machined surface. Diffusion of Ni, Fe, and Cr into SiC whiskers was found to degrade them and facilitate adhesion. Chemical wear mechanisms were found to be responsible for degradation and decomposition of whiskers and formation of tribolayer on tool surfaces, which in turn was related to the reduced adhesion of Alloy 718 on the tool. Cracking on the tool rake and localized plastic deformation were found to further accelerate tool deterioration.

A hybrid algorithm to optimize cutting parameter for machining GFRP composite using alumina cutting tools

In this paper, two different evolutionary algorithm-based neural network models were developed to optimise the unit production cost. The hybrid neural network models are, namely, genetic algorithm-based neural network (GA-NN) model and particle swarm optimization- based neural network (PSO-NN) model. These hybrid neural network models were used to find the optimal cutting conditions of Ti(C,N) mixed alumina-based ceramic cutting tool (CC650) and SiC whisker-reinforced alumina- based ceramic cutting tool (CC670) on machining glass fibre-reinforced plastic (GFRP) composite. The objective considered was the minimization of unit production cost subjected to various machine constraints. An orthogonal design and analysis of variance was employed to determine the effective cutting parameters on the tool life. Neural network helps obtain a fairly accurate prediction, even when enough and adequate information is not available. The GA-NN and PSO-NN models were compared for their performance. Optimal cutting conditions obtained with the PSO-NN model are the best possible compromise com- pared with the GA-NN model during machining GFRP composite using alumina cutting tool. This model also proved that neural networks are capable of reducing uncertainties related to the optimization and estimation of unit production cost.

Deformation and failure processes during tensile creep of fibre and whisker reinforced SiC/Al2O3 composites

Abstract The tensile creep and creep fracture properties in air from 1473 to 1673 K are reported for an alumina matrix composite, reinforced with interwoven bundles of silicon carbide (Nicalon™) fibres aligned parallel and normal to the stress axis. The creep strength of this 0/90° SiC f /Al 2 O 3 composite is determined by the longitudinal fibres, with creep of the fibres accompanied by crack development in the weak porous matrix. Oxygen penetration then promotes failure of the fibres bridging the developing cracks, with creep fracture occurring when a crack becomes long enough to cause sudden failure by fibre pull-out. These behaviour patterns are discussed in relation to tensile creep data available for SiC whisker-reinforced alumina, allowing comparisons to be made between the effects of whisker and fibre reinforcement on the high-temperature creep and creep fracture characteristics of SiC/Al 2 O 3 composites.

Cyclic Fatigue Crack Growth Characteristics For SiC Whisker-reinforced Alumina Matrix Composite

It is important to characterise the fatigue damage tolerant behaviour of engineering ceramics widely used in structural applications. Cyclic fatigue crack growth tests were conducted for SiC whisker-reinforced alumina matrix composites in order to clarify the influence of whisker reinforcement on cyclic fatigue crack growth resistance for alumina ceramics.

An Investigation of the Role of Processing in the High Temperature Creep of Whisker-Reinforced Alumina Composites

Abstract The creep properties of polycrystalline alumina at elevated temperatures may be improved by incorporating SiC whiskers into the matrix to form a composite material. However, the present experiments show that the creep properties of SiC whisker-reinforced alumina depend upon the morphology of the whiskers contained within the alumina matrix and therefore upon the processing procedure used to fabricate the composite. A comparison is made between creep data obtained from a conventionally processed alumina composite reinforced with SiC whiskers and the results obtained under similar experimental conditions on a composite fabricated using a dispersion processing procedure.

R-Curve Response Of Silicon Carbide Whisker-Reinforced Alumina: Microstructural Influence

AbstractRising fracture resistance with crack extension (R-curve response) can lead to improvements in the mechanical reliability of ceramics. To understand how microstructures influence the R-curve behavior, direct observations of crack interactions with microstructural features were conducted on SiC whisker-reinforced alumina. The contribution of the dominant toughening mechanisms to the R-curve behavior of these composites is discussed using experimental and theoretical studies.

Examination of fracture surfaces of SiC whisker-reinforced alumina after high temperature creep deformation

Examination of fracture surfaces of SiC whisker-reinforced alumina after high temperature creep deformation

Residual stresses in a SiC whisker-reinforced alumina composite by high-temperature X-ray diffraction

Residual strains and microstresses are evaluated for both phase of a hot-pressed, fine-grained α-alumina reinforced with 25 wt% (29 vol%) single-crystal silicon carbide whiskers at temperatures from 25 to 1000 °C. The sample was maintained in a nonoxidizing environment while measurements of the interplaner spacing of alumina (146) and SiC (511 + 333) were made using X-ray diffraction methods. The residual strains were profiled at temperature increments of 250 °C from which the corresponding microstresses were calculated. Linear extrapolation of the SiC ε33 profile indicates that the strains are completely relaxed at a temperature of approximately 1470 °C. These residual stress relaxation results suggest that elevated temperature toughness and fracture strength of this composite may result from cooperative mechanisms.

High temperature fracture of a SiC whisker-reinforced alumina in air and vacuum

Abstract The fracture behaviour of a SiC w /alumina composite was studied between room temperature and 1320°C in vacuum by four-point bend testing of specimens pre-cracked by bridge indentation. The results are compared to those of earlier work on a similar material in air. In vacuum, the composite begins to exhibit stable crack growth (SCG) and increasing apparent toughness at about 1100°C. Between 1100 and 1320°C, the extent of SCG increases and the toughness rises to almost three times the room temperature fracture toughness. The toughness enhancement is due mainly to such processes as crack deflection, crack branching, microcrack zone formation and crack tip blunting rather than any temperature-related increase in intrinsic fracture resistance of the material. In air, the fracture processes are similar to those in vacuum but the toughness increase is delayed by about 100°C and the level of toughening is lower. These differences arise from the formation of a glass phase in air which delays microcrack formation but reduces the resistance to stable crack growth.

Effect of carbon coating on mechanical strength of SiC whisker-reinforced alumina composites

Microstructure and mechanical response of alumina matrix reinforced with carbon coated and uncoated SiC whiskers were examined. Carbon coating was used in order to modify the interface and possibly relax the residual thermal stresses developed on cooling from the hot-pressing temperature. Crack propagation behaviour and interfacial properties of the composite were studied using scanning and transmission electron microscopy. Resistance to fracture was assessed by using four point bend test and failure load was determined using four point chevron notched beam and double cantilever beam specimens. The improvement in fracture resistance in carbon coated whisker composites was attributed to whisker pull out, crack bridging and crack deflection.

Tribological characterization of alumina and silicon carbide under lubricated sliding

The aim of this study was to investigate the tribological properties of four different advanced ceramics - monolithic alumina, SiC whisker-reinforced alumina, monolithic silicon carbide and SiSiC-during lubricated sliding. Advanced techniques of electron microscopy and spectroscopy have been used to characterize the materials before and after testing. Tests have been performed where two flat discs were rotated against each other under closed contact in an environment of oil and water. The main wear mechanisms of the four ceramics are identified and discussed. What clearly emerges from these studies is the much more reliable performance of the silicon carbides than the aluminas. The silicon carbides have a low wear rate where microfracture and oxidation are the main deteriorating mechanisms. The capability to maintain smooth surfaces and thus also a high degree of hydrodynamic lubrication is largely due to the potential of the water to dissolve formed wear debris. The main wear mechanism of the aluminas is surface fracture. The rough fracture surfaces and the fact that the wear fragments form a discontinuous surface film will reduce the effect of the lubrication, thus accelerating the wear. Furthermore, a deformation layer with microcracks develops in the contact which decreases wear resistance.

An Analysis of the Wear Behavior of SiC Whisker-Reinforced Alumina from 25° to 1200°C

A model is described for predicting the wear behavior of whisker reinforced ceramics. The model was successfully applied to a silicon carbide whisker reinforced alumina ceramic composite subjected to sliding contact. The model compares the friction forces on the whiskers due to sliding, which act to pull or push them out of the matrix, to the clamping or compressive forces on the whiskers due to the matrix, which act to hold the whiskers in the composite. At low temperatures, the whiskers are held strongly in the matrix and are fractured into pieces during the wear process along with the matrix. At elevated temperatures differential thermal expansion between the whiskers and matrix can cause loosening of the whiskers and lead to pullout during the wear process and to higher wear. The model, which represents the combination of elastic stress analysis and a friction heating analysis, predicts a transition temperature at which the strength of the whiskers equals the clamping force holding them in the matrix. Above the transition the whiskers are pulled out of the matrix during sliding, and below the transition the whiskers are simply fractured. The existence of the transition gives rise to a dual wear mode or mechanism behavior for this material which was observed in laboratory experiments. The results from this model correlate well with experimentally observed behavior indicating that the model may be useful in obtaining a better understanding of material behavior and in making material improvements.

Scanning electron acoustic microscopy of indentation-induced cracks and residual stresses in ceramics

The ability of scanning electron acoustic microscopy (SEAM) to characterize ceramic materials is assessed. SEAM images of Vickers indentations in SiC whisker-reinforced alumina clearly reveal not only the radial cracks, the length of which can be used to estimate the fracture toughness of the material, but also reveal strong contrast, interpreted as arising from the combined effects of lateral cracks and the residual stress field left in the SiC whisker-reinforced alumina by the indenter. The strong contrast is removed after the material is heat treated at 1000 °C to relieve the residual stresses around the indentations. A comparison of these observations with SEAM and reflected polarized light observations of Vickers indentations in soda-lime glass both before and after heat treatment confirms our interpretation of the strong contrast.

Temperature Dependence of Strengthening by Whisker Reinforcement: SiC Whisker-Reinforced Alumina in Air

The high fracture toughness of alumina reinforced with 20 vol% SiC whiskers is maintained at temperatures up to {approx} 1,100{degree}C in air. The fracture strength values ({sigma}{sub f}) decrease slowly with increasing temperature to 1,100{degree}C; however, when subjected to an applied stress (= 2/3 {sigma}{sub f}) for periods of up to 1,000 h in air at 800{degree}, 1,000{degree}, or 1,100{degree}C, the subsequent fracture strength values (retained strengths) at these temperatures increase with increasing exposure times. On the other hand, above 1,100{degree}C, the fracture strength values decrease while the apparent toughness value increases. At 1,200{degree}C, samples first subjected to applied stresses of {le} 310 MPa for up to {approx} 240 h exhibit a loss in retained strengths with increasing exposure time. The temperature dependence of the mechanical behavior is shown to be related to oxidation reactions in these composites with creep phenomena contributing to the loss in strength and increased toughness above 1,100{degree}C.

The Mechanical Properties at High Temperatures of SiC Whisker-Reinforced Alumina

AbstractCreep tests were conducted at temperatures from 1673 to 1823 K on samples of Al2O3 containing 9.3, 18 and 30 vol % SiC whiskers. The results show that the strains to failure tend to decrease and the stress exponents for the steady-state creep rates tend to increase with increasing volume percentages of SiC whiskers.