Temperature-Dependent Indentation Behavior of Transformation-Toughened Zirconia-Based Ceramics

Indentation behavior of Ce-TZP, Y-TZP, and Mg-PSZ between room temperature and 1300°C was investigated. Hardness decreased with increasing temperature for all three materials, but indentation cracking increased with increasing temperature. The opposing temperature dependences are discussed in terms of dislocation and transformation plasticity.     

Effect of humidity on friction,wear, and plastic deformation during nanoscratch of soda lime silica glass

Physical flaws and defects on glass surfaces are known to reduce the mechanical strength and chemical durability of glass. The formation of surface defects depends not only on the mechanical conditions of the physical contact but also on the environment in which the contact is made. In this study, the nanoscratch behavior of soda lime silica (SLS) glass was investigated in 10% and 60% relative humidity (RH) conditions. Based on the evolution of friction and scratch depth, the deformation of SLS glass surface could be divided into four regimes: elastic deformation and recovery (E), RH-independent mild plastic deformation (P-1), RH-dependent intermediate plastic deformation (P-2), and RH-independent severe plastic formation (P-3). It is quite surprising to observe that plastic deformation of the glass surface has dependence on RH of the environment (outside the glass) because plastic deformation is the process occurring below the surface (inside the glass) by the externally applied load. From this result, it can be inferred that frictional energy dissipation mode at the sliding interface, which is a function of adsorbed water molecules, influences the subsurface deformation mode. Although friction, wear, and subsurface deformation/damage are all coupled, there is no direct one-on-one correlation among them.     

Contact/scratch-induced surface deformation and damage of copper-graphite particulate composites

The elastoplastic surface deformation and damage under frictional sliding contact of copper-graphite particulate composites with Cu-content ranging from 0 to 40 vol% are examined in indentation and scratch tests using a load controlled test system. The contact areas in indentation and scratch tests are estimated with the Field-Swain approximation. The characteristic material parameters of the elastic modulus E′, yield stress Y, interfacial shear strength s, and the scratch resistance are discussed in relation to the Cu-content of the composites. The microscopic mechanisms and processes for the surface deformation and damage induced by frictional sliding contact are also examined. With the increase in the normal contact load, the scratch-induced surface deformation and damage are transiently followed with the sequential four stages: (I) the elastoplastic grooving, (II) plastic plowing, (III) microcracking, and (IV) the inter- and intra-fractures and chipping of graphite particles. The Cu-content in the composite plays the key role in controlling the characteristic contact pressures for these transitional deformation/damage processes.     

Statistical fatigue strength evaluation and inelastic deformation generated during static and cyclic loadings in Ce-TZP/alumina nanocomposite: part 2 – in water environment

The statistical fatigue strength evaluations in water of an intragranular type Ce-TZP/Al₂O₃ nanocomposite (Ce-TZP/A-N) were investigated in comparison to 3Y-TZP. The isothermal t-m transformation which causes low temperature aging degradation (LTAD) did not occur during fatigue tests for both materials. The strength degradations under static and cyclic loadings of Ce-TZP were fairly small compared to those of 3Y-TZP. The life shortening in water of 3Y-TZP was slight, and Ce-TZP/A-N maintained comparable fatigue lives found in air. Large inelastic deformations (converted strain = 0.06–0.23%) were observed in the non-failure fatigued specimens after static and cyclic loadings. The amount of inelastic deformations and relevant transformed monoclinic contents in water were smaller than those in air at equivalent stress level. In contrast, the volume fractions of the monoclinic phase in water were slightly higher than those in air at equivalent strain level. For 3Y-TZP, no inelastic deformation was identified.     

Fracture mechanics of sharp scratch strength of polycrystalline alumina

The strength of a polycrystalline alumina containing controlled scratches introduced by translated sharp contacts is investigated and described by a multiscale fracture mechanics model. Inert strength measurements of samples containing quasi‐static and translated Vickers indentation contacts showed that scratches degraded the strength at normal contact loads an order of magnitude less than those for quasi‐static indentation. The fracture mechanics model developed to describe strength degradation by scratches over the full range of contact loads included toughening effects by crack‐wake bridging at the microscale and lateral crack‐based residual stress relaxation effects at the mesoscale. A critical element of the model is the nonlinear scaling of the residual stress field of a scratch with the normal contact load acting during scratch formation. The similarities and differences in the scratch model in comparison with prior indentation‐strength fracture mechanics models are highlighted by parallel development of both. Central to the scratch model is the use of easily controlled normal contact load as the scratch‐strength measurement variable. Scratch length and orientation are shown to have significant effects on strength. The distributions of scratch widths controlling the intrinsic strengths of as‐received samples are determined and agreement with the observed scratch dimensions is demonstrated.     

Tetragonal-to-Monoclinic Transformation in Mg-PSZ Studied by in Situ Neutron Diffraction

The deformation of 9.4 mol% magnesia-partially-stabilized zirconia under compressive loads up to 1225 MPa was studied using mechanical testing with in situ neutron diffraction. The material shows obvious plastic deformation at applied stresses in excess of an estimated critical stress of 925 ± 20 MPa. Most of the accumulated strain occurred by transient room-temperature creep. Plastic deformation was associated with considerable stress-induced tetragonal-to-monoclinic transformation. The volume change calculated from the strain gauges correlates well with the amount of t → m transformation observed. Unlike previous studies of Ce-TZP and Y-TZP, ferroelasticity was not observed, nor was the t → o transformation observed. Minor microstructural changes were noted, including an increase in the root mean square internal strain of 0.05%, commensurate with an increase in internal stress of ∼100 MPa. It would appear that transformation selectivity was exercised with the transformation occurring first in tetragonal crystallites favorably oriented to the applied stress. The stress-induced monoclinic phase therefore exhibits a strong preferred orientation. Comparison is made with the other commercially interesting zirconia ceramics, Ce-TZP and Y-TZP, which have been studied using the same techniques.     

A New Analytical Model for Estimation of Scratch-Induced Damage in Brittle Solids

Scratch tests are of fundamental interest both for understanding machining-induced damage and for evaluating the scratch resistance of brittle materials. An improved blister field model for the scratch process is proposed where the blister field strength is explicitly determined in terms of the material properties, loading conditions, and geometry of the scratch tool. Additionally, one new expanding cylindrical cavity model is implemented to estimate the plastic zone size surrounding the scratch groove. A quantitative evaluation of the damage zone size is conducted by combining the above two models. The predicted damage zone sizes are in good agreement with the results available elsewhere in literature.     

Effect of the substrate and interface on micro-scratch deformation of epoxy-polyester powder coatings

The role of the interface on the deformation response in scratch tests of epoxy-polyester films deposited by electrostatic spraying is investigated. A comparative study of the scratch deformation behaviour of films deposited on micro- and macro-corrugated rigid substrates and on ‘soft’ silicon sub-layers is made. Scratch deformation parameters were evaluated by contact gauge inductive profilometry, whilst morphological examinations of the residual scratch patterns were performed by electron microscopy.     

Effect of scratch velocity on scratch behavior of injection-molded polypropylene

Scratch tests of polypropylene injection moldings were conducted with a progressive load scratch test according to ISO 19252. Effect of scratch velocity on scratch visibility and damage initiation was investigated. The results showed that the critical normal load for onset of scratch visibility was independent of scratch velocity in the range from 1 to 10 mm/s. However, the critical normal load decreased with increase of scratch velocity higher than 10 mm/s. The other scratch damage transitions such as onset of fish-scale and cutting pattern, the critical normal load also decreased with increase of scratch velocity. A correlation between scratch behavior and subsurface deformation was observed by polarized optical microscope. The formation of yielded zone under scratch groove was clearly observed at onset of scratch visibility. It was found that the yielded zone size becomes shallower at higher scratch velocity. The results suggested that localized stress was generated near the surface at higher scratch velocity. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Microstructural changes in 3Y-TZP induced by scratching and indentation

Microstructural changes produced in 3Y-TZP by contact with a Vickers indenter during indentation and scratch tests have been studied by μ-Raman spectroscopy and transmission electron microscopy. In the scratch test, the monoclinic phase distribution under the scratch track is maximal at the surface, close to the edges of the groove, where the pile-up is formed by the flow of deformed material. This distribution coincides with the plastic deformed zone observed by electron microscopy. For indentation, the highest concentration of monoclinic phase is found below the imprint at a depth approximately equal to its semi-diagonal, while plastic deformation is concentrated in a narrow region of less than 1 μm just below the indenter tip. The present results show that the microstructural changes induced by the scratch test are similar to those induced during grinding.     

Statistical fatigue strength evaluation and inelastic deformation generated during static and cyclic loading in Ce-TZP/alumina nanocomposite: Part 1—in air environment

The statistical fatigue strength evaluations of an intragranular type Ce-TZP/Al₂O₃ nanocomposite (Ce-TZP/A-N), such as its initial strength, static and cyclic fatigue lives, and its dispersion, were investigated in comparison to 3Y-TZP. The strength degradations during static and cyclic loading of Ce-TZP/A-N were fairly small, and the dispersions of the fatigue life were also quite small compared to those of 3Y-TZP, especially for the case of cyclic loading. In addition, fairly large inelastic deformations (converted strain ≈0.1–0.3%) were observed in the non-failure fatigued specimens after both static and cyclic loading. The amount of inelastic deformations was generally higher under the static loading than under cyclic loading, and increased with increasing the applied stress. In contrast, no inelastic deformation was identified for 3Y-TZP. By means of X-ray diffraction analysis, a good correlation between the amount of inelastic strain and the transformed monoclinic content was recognized for both static and cyclic loading.     

Microstructure and Bending Strength of 3Y-TZP/12Ce-TZP Ceramics Fabricated by Liquid-Phase Sintering at Low Temperature

With the addition of 1 wt% of MgO–Al₂O₃–SiO₂ glass as a sintering aid, 3Y-TZP/12Ce-TZP ceramics (composed from a mixture of 3Y-TZP and 12Ce-TZP powder) have been fabricated via liquid-phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y-TZP is almost unaffected, whereas that of Ce-TZP is inhibited. MgO·Al₂O₃ spinel and an amorphous phase that contains Al₂O₃ and SiO₂ (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y-TZP/12Ce-TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y-TZP ceramics without Ce-TZP particles. Ce-TZP grains and MgO·Al₂O₃ spinel in 3Y-TZP/12Ce-TZP ceramics may impede crack growth, and the bending strength is enhanced.     

Scratch hardness and deformation maps for polycarbonate and polyethylene

This paper presents results obtained from the scratching of an ultrahigh molecular weight polyethylene (UHMWPE) and a polycarbonate (PC). The data are used to obtain various surface mechanical properties such as the hardness and also the prevailing deformation mechanisms. Scratch results are reported for the case of rigid conical indenters for various tip included angles, bulk temperatures, scratch velocities, and applied normal loads. Scanning electron microscopy (SEM) and laser profilometry data are used to study the surface deformation and damage mechanisms, and to assess the topography of the surfaces after scratching. Deformation maps are provided for these polymers under different experimental conditions, which describe the various deformation characteristics. In general, these polymers show both increasing and decreasing trends for the scratch hardness values with variation of cone angle, (4qW/ηd²; where W is the normal load, d the width of the residual scratch, and q is a characteristic contact parameter, which ranges between 1 and 2). The scratch velocity, which governs the imposed strain rate, imparts an increasing effect on the hardness values, whereas a higher bulk temperature of the material decreases the scratch hardness. The measured responses of the surface properties of these polymers are shown to greatly depend upon the kind of deformation mechanism prevalent during the scratching and associated material removal processes.     

Strength of structural ceramics after exposure to sodium sulfate

A variety of advanced structural ceramics were treated in air at 1000°C for 500 h in the presence of various amounts of sodium sulfate. Room temperature flexure strength was determined before and after treatments. Results show that Ce-TZP and alumina are resistant to strength degradation after exposure to sodium sulfate, but Si₃N₄ (NC-132) and, to a lesser degree, Y-TZP are not.

The addition of sodium sulfate also has an adverse effect on the high temperature performance of the Y-TZP, by reducing its load carrying capabilities between 900 and 1000°C. For the Ce-TZP, alumina and even the Si₃N₄ there appears to be no change in the high temperature performance when sodium sulfate is present.     

Effect of grain size on Hertzian contact damage in 9 mol% Ce-TZP ceramics

The Hertzian contact damage in 9 mol% Ce-TZP ceramics with different grain sizes has been investigated. Single-cycle tests were conducted on materials of four grain sizes, 1.1, 1.6, 2.2 and 3 μm. The indentation stress–strain curves for all materials show striking nonlinearity and deviation from the Hertzian elastic response, illustrating a significant quasi-plastic component in the contact damage response. Subsurface damage patterns for these four materials are compared and contrasted using a bonded-interface sectioning technique. The transformation and deformation behaviour, characterised using optical and scanning electron microscopy, of the surface and subsurface regions revealed extensive deformation and compression-driven subsurface damage in the materials. Acoustic emission was used as a complementary technique in order to identify the damage processes during a load–unload cycle. Contact deformation and radial bands extending from the indent impressions due to autocatalytic tetragonal–monoclinic transformation are evident in all except the finest grained (1.1 μm) material. Irrespective of grain size there is no evidence of ring or cone cracking with all material showing hemispherical subsurface damage or yield zones resulting from the stress-induced tetragonal–monoclinic (t–m) transformation with extensive distributed microcracking within these areas for the 1.6, 2.2 and 3 μm grain-size materials.     

Model for Cyclic Fatigue of Quasi-Plastic Ceramics in Contact with Spheres

A model of contact damage accumulation from cyclic loading with spheres and ensuing strength degradation in relatively tough, heterogeneous ceramics is developed. The damage takes the form of a quasi-plastic zone beneath the contact, consisting of an array of closed frictional shear faults with attendant "wing" microcracks at their ends. Contact fatigue takes place by attrition of the frictional resistance at the sliding fault interfaces, in accordance with an empirical degradation law, allowing the microcracks to extend. At large numbers of cycles or loads the microcracks coalesce, ultimately into radial cracks. Fracture mechanics relations for the strength degradation as a function of number of cycles and contact load are derived. Indentation–strength data from two well-studied coarse-grain quasi-plastic ceramics, a micaceous glass-ceramic and a silicon nitride, are used to evaluate the model. Comparative tests in static and cyclic contact loading confirm a dominant mechanical component in the fatigue. At the same time, the presence of water is shown to enhance the fatigue. The model accounts for the broader trends in the strength degradation data, and paves the way for consideration of key variables in microstructural design for optimum fatigue resistance.     

Influence of processing variables on scratch and shrinkage behaviors and translucency of dental zirconia

Scratch tests were performed on porous 3 mol% Y₂O₃-stabilized zirconia dental blocks to relate compaction processing and partial sintering temperature to shrinkage and machinability of the blocks and microstructures and transmittance after full sintering. Scratch hardness of the blocks varied with increasing loads, and the variation was related to the sequential events of densification, densification and cleaving, disruption of the densified region, and chipping and longitudinal cracking. The shrinkage during final sintering was inversely proportional to the compact pressure and temperature with compact pressure having a greater impact. In contrast, the transmittance of fully sintered blocks depended largely on the partial sintering temperature because it governed the number and size of pores after completion of the sintering. Based on the influence of the variable on scratch hardness, a scratch response measure that possibly reflects the machinability of porous CAD/CAM blocks was proposed.     

Microstructure, Flaw Tolerance, and Reliability of Ce-TZP and Y-TZP Ceramics

Ce-TZP and Y-TZP ceramics were heat-treated for various times and temperatures in order to vary the microstructure. Flaw tolerance was investigated using the indentation–strength test. Reliability was quantified using conventional two-parameter Weibull statistics. Some Ce-TZP specimens were indented at slightly elevated temperatures where no transformation was observed. Results indicated that the Ce-TZP specimens were extremely flaw tolerant, and showed a relatively high Weibull modulus that scaled with both R -curve behavior and flaw tolerance. Y-TZP, on the other hand, with very little if any R -curve behavior or flaw tolerance, had a low Weibull modulus. The results also show that flaw history, i.e., whether or not a transformation zone exists along the wake of the crack, has a significant influence on strength. Strength was much less dependent on initial crack size when the crack had an associated transformation zone, whereas strength was highly dependent on cracks typical of natural processing defects. It is argued that the improvement in reliability, flaw tolerance, and dependence on flaw history are all ramifications of pronounced R -curve behavior.     

Contact damage in a Ce-TZP/Al2O3 nanocomposite

The damage tolerance of a nanocomposite based on Ce-TZP and 30 vol% Al₂O₃ has been studied under monotonic contact with a spherical indenter. The results are compared with those previously known for commercial 3Y-TZP zirconia. It is concluded that the minimum load for ring crack appearance is similar in both ceramics. However, in the nanocomposite the ring cracks penetrate much less into the bulk, because of its higher fracture toughness. Finally, the stress-induced phase transformation of the zirconia component was quantified and mapped by micro-Raman spectroscopy.