Brittle Chipping Research Articles

Surface Morphology Evaluation and Material Removal Mechanism Analysis by Single Abrasive Scratching of RB-SiC Ceramics

Reaction-Bonded Silicon Carbide (RB-SiC) ceramics possessing excellent mechanical and chemical properties, whose surface integrities have an essential effect on their performance and service life, have been widely used as substrates in the core parts of aerospace, optics and semiconductors industries. The single abrasive scratching test is considered as the effective way to provide the fundamental material removal mechanisms in the abrasive lapping and polishing of RB-SiC ceramics for the best surface finish. In this study, a novel single abrasive scratching test with an increasing scratching depth has been properly designed to represent the real abrasive lapping and polishing process and employed to experimentally investigate the surface integrity regarding different scratching speeds. Three typical and different material removal stages, including the ductile mode, ductile–brittle transition mode and brittle mode, can be clearly distinguished and it is found that in the ductile material removal stage by increasing the scratching speed would inhibit the plastic deformation and improve its surface integrity. It is also found that in the ductile–brittle transition and brittle material removal stages, to increase the scratching speed would inhibit the plastic deformation due to the fast scratching speed that limits the time of plastic deformation on the target, but it also results in the increased length of lateral cracks with the increased scratching speed which can reflect that the size of brittle chips, like brittle fractures and large grain fragmentations, increases as the scratching speed increases. It can provide the references for the optimization of the abrasive lapping and polishing of RB-SiC ceramics with high efficiency and surface quality.

Investigation of ductile/brittle chip formation zone in the context of manufactured geometry with different CAM paths strategies

For maintenance conform processing of brittle materials (ceramics, glass), the resulting chips are not continuous, as in the case of soft and tough materials, it is called a brittle chip removal range. In this machining zone, the quality of the produced surface may decline (appears the cracks and pitting of the material) and the tool life also decreases. When defining the cutting technology (technological parameters, tool edges, tool cooling / lubrication, etc.), chip removal must be carried out in the ductile range. However, due to tool wear, this range may cross the transition and then it can work in the brittle chip formation zone. In this paper, the machinability of zirconia oxid ceramics has been investigated. The aim of the research is to find out the correlation between the wear of the cutting tool, the volume change of the manufactured pockets and the transitions between the chip formation ranges measured by cracks and pitting deformations on the edge of the machined material. It was observed that they overlap to a large extent, consequently, these observed phenomena can be easily applied for tool wear and quality monitoring for realising condition-based maintenance, it is the main scientific contribution of the paper.

Моделирование съема припуска в зоне контакта при внутреннем шлифовании хрупких неметаллических материалов

Introduction. Finishing operations, in particular, cylindrical grinding, essentially form the quality parameters of products, its performance characteristics and functional suitability. At the same time, the cost of grinding work increases significantly in comparison with grinding metals, reaching an average of 20 ... 28% of the total cost of manufacturing products. The selection of the optimal parameters of the technological system based on the process simulation can improve the reliability, productivity and economic efficiency. To describe the processing of brittle nonmetallic materials, empirical dependences are mainly used, and the existing analytical models do not take into account the stochastic nature of the grinding operation and the combination of microcutting and brittle chipping when removing particles of brittle nonmetallic material and wear of the surface of the grinding tool. Purpose of the work: simulation of stock removal in the contact zone during internal grinding of brittle non-metallic materials. The task is to study the features and patterns of change in the probability of material removal when the treated surface comes into contact with an abrasive tool. In the work, the theoretical and probabilistic models are obtained, allowing to reveal the patterns of material removal in the contact zone. The models make it possible to trace the regularities of the interaction of cutting and piercing grains on the surface of the workpiece and the process of removing the allowance in the contact zone due to a combination of the phenomena of microcutting and brittle chipping, considered as a random event. The research methods are mathematical and physical simulation using the basic provisions of the theory of probability, the laws of distribution of random variables, as well as the theory of cutting and the theory of a deformable solid. Results and discussion. Data are obtained that provide a clear illustration of the patterns of material removal along the contact zone at various levels. Analysis of the results obtained shows that the peripheral speed of the tool and the rotation speed of the workpiece, which are directly included in the equation for calculating the probability of material removal, significantly affect the rate of material removal. The cross feed also has a significant effect on stock removal. A qualitative picture of the change in the probability of material removal in the contact zone during grinding of holes in brittle nonmetallic materials is obtained. The obtained patterns of change in the probability of material removal when the machined surface is in contact with an abrasive tool and analytical dependences are valid for a wide range of grinding modes, tool characteristics and other technological factors.

Вероятностная модель удаления поверхностного слоя при шлифовании хрупких неметаллических материалов

Introduction. The final quality of products is formed during finishing operations, which include the grinding process. It is known that when grinding brittle materials, the cost of grinding work increases significantly. It is possible to reduce the scatter of product quality indicators when grinding brittle materials, as well as to increase the reliability and efficiency of the operation, by choosing the optimal parameters of the technological system based on dynamic models of the process. However, to describe the regularities of the removal of particles of a brittle non-metallic material and the wear of the surface of the grinding wheel in the contact zone, the known models do not allow taking into account the peculiarities of the process in which micro-cutting and brittle chipping of the material are combined. Purpose of the work: to create a new probabilistic model for removing the surface layer when grinding brittle non-metallic materials. The task is to study the laws governing the removal of particles of brittle non-metallic material in the contact zone. In this work, the removal of material in the contact zone as a result of microcutting and brittle chipping is considered as a random event. The research methods are mathematical and physical simulation using the basic provisions of the theory of probability, the laws of distribution of random variables, as well as the theory of cutting and the theory of a deformable solid. Results and discussion. The developed mathematical models make it possible to trace the effect on material removal of the overlap of single cuts on each other when grinding holes in ceramic materials. The proposed dependences show the regularity of stock removal within the arc of contact of the grinding wheel with the workpiece. The considered features of the change in the probability of material removal upon contact of the treated surface with an abrasive tool and the proposed analytical dependences are valid for a wide range of grinding modes, wheel characteristics and a number of other technological factors. The obtained expressions make it possible to find the amount of material removal also for schemes of end, flat and circular external grinding, for which it is necessary to know the amount of removal increment due to brittle fracture during the development of microcracks in the surface layer. One of the ways to determine the magnitude of this increment is to simulate the crack formation process using a computer. The presented results confirm the prospects of the developed approach to simulate the processes of mechanical processing of brittle non-metallic materials.

An Experimental and Finite Element Approach for a Better Understanding of Ti-6Al-4V Behavior When Machining under Cryogenic Environment.

Due to increasing demand in manufacturing industries, process optimization has become a major area of focus for researchers. This research optimizes the cryogenic machining of aerospace titanium alloy Ti-6Al-4V for industrial applications by studying the effect of varying the nozzle position using two parameters: the nozzle’s separation distance from the tool–chip interface and its inclination angle with respect to the tool rake face. A finite element model (FEM) and computational fluid dynamics (CFD) model are used to simulate the cryogenic impingement of cryogenic carbon dioxide on the tool–workpiece geometry. Experiments are conducted to evaluate cutting forces, tool wear, and surface roughness of the workpiece, and the results are related to the CFD and FEM analyses. The nozzle location is shown to have a significant impact on the cutting temperatures and forces, reducing them by up to 45% and 46%, respectively, while the dominant parameter affecting the results is shown to be the separation distance. Cryogenic machining is shown to decrease adhesion-diffusion wear as well as macroscopic brittle chipping of the cutting insert compared to dry turning, while the workpiece surface roughness is found to decrease by 44% in the case of cryogenic machining.

Processing of T6 heat-treated Al-12Si-0.6Mg alloy

ABSTRACTIn this study, ternary Al-12Si-0.6Mg material was manufactured by gravity die casting method in induction melting furnace. Microstructure images of alloy were taken on optical microscope after T6 heat treatment. Hardness, yield and tensile strength and breaking elongation of as-cast and heat-treated materials were measured by universal methods. CNC lathe was used for cutting tests and dynamometer was used to measure cutting force. Cutting tests were performed by using different cutting speeds-CS (450-500-550 m/min), feed rates-FR (0.05–0.15-0.25 mm/rev) and constant depth of cut-DOC (1.5 mm). Uncoated (A), CVD-TiCN + TiN (B) and PVD-TiAlN+TiN (C) coated carbide inserts were selected as a cutting tool. In the microstructural observations, it was determined that the structure of the material made up of aluminum rich α, primary and eutectic silicon, δ (Al4FeSi2) and π (Al8Mg3FeSi6) phases. The heat treatment refined the phases in the structure of the alloy. In addition, it has been determined that it improves mechanical properties (hardness, yield and tensile strength) by spheroidizing silicon particles. As a result of the cutting tests, it was detected that the cutting force (CF) reduced with T6 heat treatment at all CS and FR values. The CF, BUE (Built up edge) and BUL (Built up layer) heightened with increasing FR, while it reduced with increasing CS on all cutting tools. CF, BUE and BUL were formed at least in tools A, B and C, respectively. While continuous chip formation was detected in the as-cast part, brittle chip formation was observed in the heat-treated part due to the reduction in breaking elongation of the material.

Fundamental understanding of the deformation mechanism and corresponding behavior of RB-SiC ceramics subjected to nano-scratch in ambient temperature

To get insight into nano-scale deformation behavior and material removal mechanism of RB-SiC ceramic, nanoscratch experiments were performed using a Berkovich indenter. Structure changes in chips and subsurface deformation were characterized by means of Raman spectroscopy. The result shows that the SiC phase underwent amorphization in ductile chips, while no amorphous feature can be observed in brittle chips and substrate within scratch groove. The following estimated stress surround the indenter reveals that amorphous deformation in ductile chips is governed by tangential stress (above 95 GPa), whereas the dislocations-based substrate deformation mechanism was dominated by normal stress. In the end, the effects of normal load and scratching velocity on the scratch behavior including scratch residual depth, elastic recovery and friction coefficient that related to RB-SiC ceramic deformation mechanism were also analyzed. With the increase of normal load, the deformation mechanism transfers from ductile to brittle fracture mode and cause the decrease of elastic recovery and the increase of residual depth and friction coefficient. Furthermore, the increased high density of dislocations as a result of the increased scratching velocity give rise to the increase of scratch hardness, which finally result in the increase of elastic recovery and decrease of residual depth and friction coefficients. This study contributes a new understanding of the brittle material deformation mechanism during a nano-scale scratching process.

Increased Operating Properties of Cutting Ceramics by Application of Nanostructured Multilayer Wear-Resistant Coating

This paper presents the results of the development of nanoscale multi-layered composite coatings for improving the contact performance of ceramic cutting tools. These nanocoatings help reducing brittle fracture of cutting edges and ensure a balanced wear of tool contact areas during dry high-speed machining of hardened steels. A filtered cathodic vacuum arc deposition process was used to generate the nanoscale multi-layered composite coatings (NMCC) with improved physical and mechanical properties and adhesion strength to ceramic substrate. In this work, thermodynamic criteria were used to evaluate the selected composition of NMCC, and the crystal-chemical, physical, mechanical and cutting properties and wear mechanisms were studied. Ti-(TiAl)N-(ZrNbTiAl)N coating was deposited on Al2O3, Al2O3-TiC and Si3N4 ceramic substrates. Test results showed that the wear mechanism of the coated tool was predominantly due to adhesive-fatigue processes. The development of wear was observed to be centered on the rake and flank faces of the tool without brittle chipping. The results of high-speed longitudinal turning of hardened steel X153CrMoV12 showed an increase in tool life by a factor of 1.5 against the uncoated tools and a factor of 1.3 compared with standard PVD coating Ti-(TiAl)N.

Experimental study on the erosion behavior of WC-based high-velocity oxygen-fuel spray coating

In oil and gas industries, erosion wear presents a major problem in the hydraulic fracturing process and severely reduces the erosion life of tools. Tungsten carbide (WC)-based high-velocity oxygen-fuel (HVOF) thermal spray coating is widely applied to improve surface properties. However, the erosion wear mechanism of this coating in liquid–solid two-phase flows is not clearly understood. Therefore, in this study, the HVOF thermal spray WC-based coating was characterized, and the erosion behavior of the coating was studied in a wet erosion test rig. Results showed that the experimental parameters, namely, attack angle, impact speed, particle size, and erosion time, exerted significant effects on the erosion rates of the specimens. The coated specimens exhibited higher hardness and better erosion wear resistance than the uncoated substrate due to the hard phase particles of the coating. The surfaces of the coated specimens showed microcutting, craters, and brittle chipping, as a visible manifestation of material removal. Furthermore, the erosion wear mechanism of the deposited coating was established by analyzing the eroded morphologies. The findings of this study can help enrich the understanding of the erosion wear mechanism of WC coatings in liquid–solid two-phase flows.

Power cycle reliability of Cu nanoparticle joints with mismatched coefficients of thermal expansion

The power cycle reliability of Cu nanoparticle joints between Al2O3 heater chips and different heat sinks (Cu-40wt.%Mo, Al-45wt.%SiC and pure Cu) was studied to explore the effect of varying the mismatch in the coefficient of thermal expansion (CTE) between the heater chip and the heat sink from 4.9 to 10.3ppm/K. These joints were prepared under a hydrogen atmosphere by thermal treatment at 250, 300 and 350°C using a pressure of 1MPa, and all remained intact after 3000cycles of 65/200°C and 65/250°C when the CTE mismatch was less than 7.3ppm/K, despite vertical cracks forming in the sintered Cu. When the CTE mismatch was 10.3ppm/K, the Cu nanoparticle joint created at 300°C endured the power cycle tests, but the joint created at 250°C broke by lateral cracks in the sintered Cu after 1000cycles of 65/200°C. The Cu nanoparticle joint created at 350°C also broke by vertical cracks in the heater chip after 1000cycles of 65/250°C, suggesting that although sintered Cu can be strengthened to tolerate the stress by increasing the joint temperature, this eventually causes the weak and brittle chip to fracture through accumulated stress. The calculation results of stresses on the heater chip showed that the stress can be higher than the strength of Al2O3 when the CTE mismatch is 10.3ppm/K and the Young's modulus of the sintered Cu is higher than 20GPa, suggesting that the heater chip can be broken.

Modelling the particles impingement angle to produce maximum erosion

A semi-mechanistic, semi-empirical erosion model was developed and validated by experimental data. The novelty of the developed model is by accurately predicting the impingement angle that produce maximum erosion and relating it to measurable mechanical properties such as hardness and fracture toughness. The ratio of the target material fracture toughness (R) to its hardness (H) is the parameter which governs the erosion mechanism and consequently the angle for maximum erosion. It was noticed that for material with high R/H, i.e. ductile materials, the preferred mode is ploughing, a process analogous to rubbing in which the penetrating particle squeeze the target material out of the indentation to preform ridges, which are knocked off at subsequent impacts. The lower the ratio R/H is, a tendency to fracture mechanism occurs. The fracture mechanism is a process very similar to brittle chipping during metal cutting, where the applied force propagates cracks parallel to the surface. A simplified model, defining the horizontal force which is applied on the particle in its movement through the material, was introduced. This simplified model takes into account the relative effect of each of the ploughing and fracture mechanism on the total material removal. The material parameter which governs the relative effect of the mechanisms is R/H. Particle angularity also contributes to decreasing of the fracture mechanism, hence a governing non-dimensional number was received, R/H·deff, where deff is the effective diameter of the penetrating particle tip. Incorporating this parameter in the erosion model yielded good agreement with experimental data and was able to explain the phenomenon of different maximum erosion angle and velocity exponent for different target materials. The relation between the developed model empirical coefficients to target material mechanical properties was studied for eight different materials, ranging from soft metals to hardened stainless steel. Correlations between model coefficients to R/H were developed and exhibited good agreement with mechanistic analysis.

Experimental investigation of end milling of titanium alloys with polycrystalline diamond tools

The low thermal conductivity and high chemical reactivity of titanium alloys result in a short tool life in the milling process. This article investigates the performance of polycrystalline diamond tools in the end milling of titanium alloys (Ti6Al4V) by using small customized cutting tools. The relationship between cutting force and cutting parameters was analysed; tool life, tool wear, and causes that lead to tool failure were discussed. To analyse tool wear and cutting temperatures, residual chemical components on the cutting tool were examined with X-ray diffraction method, while surface integrity of cutting tools was inspected based on the images taken by the scanning electrical microscope. Finite element analysis models were developed to simulate the initiation of cracks under different loading cycles. Through cutting experiments, it was found that brittle chipping and fatigue were the two major modes of failure, and feed rate was the dominant factor that causes large cutting forces.

Features of wear of brittle inorganic materials during friction and abrasive machining

Laser confocal microscopy reveals fatigue cracking under the surface of silicate glass upon friction and abrasive machining. Surface cracking is also registered and its maximum depth is determined, indicating that its longitudinal cross section has an irregular profile. The stage of fatigue wear of the glass corresponding to debris nucleation shouldbe visualized. It is established that the mass wear rate and the maximum surface cracking depth are correlated: once a definite sliding velocity is reached, the cracking becomes deeper and the wear rate intensifies in both types of tests. The obtained results prove that several wear mechanism can occur simultaneously during abrasive machining of brittle inorganic materials, namely, brittle chipping, low-cycle fatigue, thermomechanical fracture, and local melting (under critical loading conditions).

An analysis of adhesive properties of brittle coatings

In a previous study [1] a pendular scratching test was used to perform single-tip abrasion under controlled conditions on massive samples. During operation a pendulum swung down and an abrasive tip, which was fixed radially to this rigid pendulum, scratched a horizontal specimen at the bottom of the trajectory. The length 2L and the maximum depth D of the arc-shaped groove produced could be varied by adjusting the radial position of the tip. A transition scratching depth D* from ductile to brittle abrasion was then defined: this transition depth was found to be correlated to the ratio Kc/H, where K c is the toughness and H the hardness. The relationship was similar to that obtained in static indentation [2, 3] and the transition depth D* was found to fit the relationship D* = ( Kc/H) 15 The incident velocity of the abrasive tip was 2 m s -1 . These scratching tests were run with half-conical indentors (apex angle 90°). The flat surface was in front of the indentor, at a right angle to the scratching direction during experiments. Scratching tests are also commonly used for testing the adhesive properties of brittle coatings [4, 5]. The applied load is increased until brittle chipping occurs, the phenomenon appears similar to that obtained on massive samples and described above. This critical load of brittle chipping is sometimes used to characterize the adhesive proper- ties of coatings. Pendular experiments were conducted in order to analyse the process of scratching of semibrittle coatings and to discern the effects of the brittleness (ratio H/Kc) from effects of the adhesive proper- ties. Tests were run with the same half-conical indentors described above and,with conical inden- tors, apex angle 90 °. The incident velocity of the abrasive tip was 2ms -I. The energy consumed during abrasion was given by the loss of potential energy of the pendulum. A particular material was used in order to observe easily the different effects: aluminium (99.9% pure) coated with commercial unsaturated polyester (UP) resin. Samples were prepared in order to obtain good or poor adhesion of the coating. The thickness of the coating was e = 0.05 ram. Tests were also run on massive samples of aluminium and polyester. A typical morphology of the surface damage is shown in Fig. 1. When the indentor scratches only the coating, a first critical depth D1 is observed. This depth D1 characterizes the onset of lateral cracking of the coating and corresponds to the transition

Erosion and strength degradation in alumina

The effect of multiparticle impact at 23 700 and 1000 °C on the erosion and strength degradation of sintered alumina was determined. Impact damage at normal incidence was characterized by numerous small pits consistent with grain boundary cracking. Neither erosion rate nor strength degradation for normal impingement depended strongly on temperature. Although erosion rate increased with increasing kinetic energy of the impacting particle, strength degradation leveled off at high kinetic energies, contrary to predictions based on elastic-plastic fracture. An alternative theory was developed on the basis of the observed damage morphology. This theory assumes that the kinetic energy of the impacting particle goes into grain boundary cracking and the subsequent grain fallout creates a hemispherical pit with an annular crack. This model correctly predicts the dependence of both erosion rate and strength degradation on the kinetic energy of the impacting particle. For impingement at low angles, impact damage was primarily by a ductile ploughing process. This caused erosion rates to be greater than that predicted on the basis of the brittle chipping mode of erosion, but strength degradation was less because the shallow elongated flaws are less severe than the pits produced by normal impact.