Influence of C doping on the fracture mode and abrasive wear of Al2O3

Previous studies have shown that the addition of SiC nanoparticles to Al₂O₃ changes the fracture mode from intergranular to transgranular and in doing so improves the wear resistance. The reason for this is not clear but a change to the grain boundary chemistry caused by impurities such as C added with the SiC may be involved. The aim of the current study was to investigate the influence of small amounts of C doping on the fracture mode and wear properties of Al₂O₃. The microstructure and properties of Al₂O₃ doped with 0 and 0.012 wt% C were studied. Al₂O₃ showed mainly intergranular fracture. The addition of 0.012 wt% C to Al₂O₃ changed the fracture mode to mainly transgranular. The wear resistance improved and the percentage of surface grains pulled out was lower compared to pure Al₂O₃.     

Study of tribological properties of polymer derived ZrB2-SiC ceramics

ZrB₂–SiC composite ceramics with different compositions (20 and 60?vol% ZrB₂–SiC, 20ZS and 60 ZS, respectively) were prepared. Wear tests were conducted on the obtained ceramics in multiple distances using ball-on-flat tribotester. Volume loss and cross-sectional profiles of samples were measured by three–dimensional (3D) profilometer to study the onset of track wear damages. Pressure–depth curves and hardness were measured by indentation to investigate defects produced in the tribo-film by the debris. The debris of 20ZS was found to be joined to the tribo-film and accumulated with distance, shifting from microcrack (＜10,000 cycles) to abrasive wear (50,000 cycles). Compared to 20ZS, lower debris accumulation of 60ZS resulted in better wear resistance, leading to thinner and more stable non-substrate regions for this sample. These differences between both samples basically resulted from different particle sizes. Fine grains were easily pulled out in the experiment, resulting in abrasive wear of the specimen. While transgranular fracture of grains and the pinning led to larger grains with less debris, the damage mode remained transgranular fracture.     

Effects of MgO and Fe2O3 additives on the microstructure and fracture properties of aluminium titanate flexible ceramics

Aluminium titanate (Al₂TiO₅, AT) flexible ceramics were prepared from Al₂O₃–TiO₂ powder system with MgO and Fe₂O₃ as additives through solid-state method. The effects of addition level of MgO and Fe₂O₃ on phase compositions, sintering behavior, microstructure and fracture properties of AT flexible ceramics were systematically investigated. The experimental results show that the introduction of additives can promote the formation of AT and improve the densification by forming solid solution. The addition of MgO could effectively refine AT grains since the formed MgAl₂O₄ spinel grains could pin at the AT grain boundary and inhibit the growth of AT grains. Conversely, the addition of Fe₂O₃ could promote the AT grain growth. And the simultaneous addition of MgO and Fe₂O₃ is beneficial to develop elongated rod-like AT grains. With that, the improved fracture properties can be obtained. Due to pining effect of spinel and better densification, the flexural strength of modified AT flexible ceramics is about 34 times higher than that of virgin. In addition, thanks to the microcracked structure and high grain aspect ratio, events of crack deflection, crack branching, grains pull-out and grains bridging are more likely to occur, leading to an increase in the flexibility by about 133%.     

The physicochemical processes in the reactions of the components of system Mg(Al1−XCrx)2O4-SiO2-iron oxide

Conclusions The phase transition in the firing of compositions based on the system Mg(Al_1–XCr_X)₂O₄-(Fe₂O₃, SiO₂) consist essentially of the formation of solid solutions of spinels and the ferrous component of the reagent, and the solution of the spinel in the iron-silica melt.In compositions with magnesia-alumina spinel the solution of the spinel in the melt is the dominant process in every case while in compositions with a magnesite-chromite spinel the dominance of a given process depends on the composition of the reagent.The solubility of spinels in an iron-silica melt depends on the composition of the spinel and the percent iron oxides in the primary melt while the total percent melt depends mainly on the percent silica in the original mixture. The solubility of high-alumina spinels in melts of Fe₂O₃ and SiO₂ is greater than that of high-chromite spinels in every case.Deceased.Translated from Ogneupory, No. 7, pp. 45–49, July, 1974.     

Fracture behaviour of magnesia and magnesia–spinel composites before and after thermal shock

Microstructural changes, as a consequence of the thermal expansion mismatch between magnesia and spinel phases, and fracture behaviour of magnesia and spinel composites have been investigated as a function of spinel content (10, 20, 30 wt.%). Fracture surfaces of magnesia showed mostly transgranular fracture; for the composites, however, the amount of intergranular fracture increased with increasing spinel content. This change in fracture behaviour is thought to be the main reason for the increase in the work of fracture, γ_WOF. The 30% spinel composite was found to exhibit both the greatest resistance to crack propagation, and the greatest resistance to thermal shock damage, with the highest retained strength after quenching.     

Synthesis and densification behaviour of magnesium aluminate spinel: Effect of Dy2O3

Magnesium aluminate spinels have been developed by reaction sintering of calcined alumina and calcined magnesia and its densification behaviour was studied in presence of Dy₂O₃. Green bar made from stoichiometric spinel composition with and without Dy₂O₃ were subjected to dilatometric study, densification study and microstructural evaluation by SEM. It was found that Dy₂O₃ additive does not have significant effect on the spinelisation but favours the densification of the spinel. Microstructure of sintered spinel without any additive is non-uniform with some exaggerated grain growth. Dy₂O₃ prevents the exaggerated grain growth and thereby helps in the densification process.     

Preparation and properties of MgAl2O4 spinel ceramics by double-doped CeO2 and La2O3

Magnesium aluminate spinel (MgAl₂O₄) ceramics are high-performance and carbon-free materials widely used in both military and civilian fields. However, it is usually challenging to densify during the solid-state sintering process. The excellent properties of some rare earth oxides have been proved to promote the densification of MgAl₂O₄ spinel ceramics. But the mechanism of promoting sintering is not clear. In the present work, MgAl₂O₄ spinel ceramics have been successfully fabricated by co-doping CeO₂ and La₂O₃ via a single-stage solid-state reaction sintering. The effects of addition amounts of CeO₂ and La₂O₃ on phase compositions, microstructures, sintering characteristics, cold compressive strength, and thermal shock resistance of as-prepared MgAl₂O₄ spinel ceramics were systematically investigated. The results show that by co-doping CeO₂ and La₂O₃ can increase the defect concentration due to the lattice distortion. This could promote the movement of Al³⁺ and Mg²⁺ at high temperature, which is beneficial to the formation of more secondary MgAl₂O₄ spinel. t-ZrO₂ with more Ce⁴⁺ filling between spinel grains could prevent the growth of grains and promote the densification, besides the new-formed LaAlO₃ that was mainly distributed along the grain boundary of the MgAl₂O₄ phase, both of which were favorable for the formation of dense microstructure of MgAl₂O₄ spinel materials. At the same time, the formation of more secondary MgAl₂O₄ spinel and sintering densification also improve the mechanical properties of spinel ceramics. La³⁺ will segregate to the spinel grain boundary, preventing grain boundary movement and absorbing the main crack's fracture energy. With 3 wt% CeO₂ and 3 wt% La₂O₃ co-doping, the bulk density of the sample increased from 3.02 g∙cm⁻³ to 3.55 g∙cm⁻³; the apparent porosity decreased from 12.21% to 9.97%; the cold compressive strength increased from 172.88 MPa to 189.54 MPa; and the residual strength retention ratio after thermal shock increased from 84.92% to 89.15%.     

Influence of the SiC grain size on the wear behaviour of Al2O3/SiC composites

Dry ceramic block-on-steel ring wear tests were performed at high loads in several Al₂O₃/20 vol.% SiC composites as a function of the SiC grain size, which ranged from 0.2 to 4.5 μm in d₅₀. The wear resistance of the monolithic alumina was radically improved by the addition of the SiC particles, reducing down to one order of magnitude wear rate. Two different behaviours were identified according to the microstructural observations on the worm surfaces: intergranular fracture and grain pull-out in the monolithic Al₂O₃, and plastic deformation and surface polishing in the composites. The wear resistance of the Al₂O₃/SiC composites increased with the SiC grain size due to their fracture toughness enhancement.     

Microstructure and phase evolution of Indian magnesite‐derived MgAl2O4 as a function of stoichiometry and ZrO2 doping

To aid development of cost‐effective sintered spinel as a refractory raw material, this paper presents an extensive analysis of microstructure and complex phase evolution of Al‐rich, Mg‐rich, and stoichiometric spinel aggregates derived from Indian magnesite and calcined alumina. Pore morphology in Al‐rich spinel was transformed upon sintering at 1650°C and corundum laths embedded in porous Al‐rich spinel matrix was formed. Stoichiometric spinel sintered at 1600°C consisted of mostly direct bonded angular equiaxed spinel grains which incorporated the impurities in solid solution. Mg‐rich spinel was composed of spinel grains with reduced angularity along with intergranular amorphous phase, small round monticellite grains, and periclase. EDS line scan revealed impurity‐free joins existed between direct bonded spinel grains. Mg‐rich spinel containing 0.65 wt% ZrO₂ formed cubic ZrO₂‐CaO‐MgO solid solution located along spinel boundaries, which reduced both intergranular amorphous phase and monticellite. This increased SiO₂ and MgO content in spinel solid solution triggering exsolution of metastable cubic forsterite manifested as split spinel peaks in XRD pattern. A 14.7% reduction in slag penetration was exhibited upon doping Mg‐rich spinel with 0.21% ZrO₂. Stoichiometric and Mg‐rich spinels attained 0.35% and 0.54% apparent porosity at 1600°C, which is better than most commercial sintered refractory spinels.     

Numerical simulation analysis and experimental validation on wear/fracture mechanisms of polycrystalline cubic boron nitride superabrasives in high-speed grinding

In this study, a two-dimensional finite element model is proposed to investigate the wear/fracture mechanisms of polycrystalline cubic boron nitride (PCBN) superabrasives in high-speed grinding process. The special geometric microstructures of PCBN grains are constructed by using the classic Voronoi tessellation technique, and cohesive elements are embedded into the geometric model of PCBN grains as the potential crack propagation paths for simulating the wear/fracture behaviours of PCBN grains under grinding loads. The effects of uncut chip thickness per grain (a_gmax) on the stress distribution characteristics and wear/fracture behaviours of PCBN grains during grinding are discussed in detail. Results show that the wear behaviour of PCBN grains during grinding mainly occurs around the grain vertex region; however, the fracture behaviour, leading to the quick failure of PCBN grains, is prone to appear around the grain–filler bonding interface, which is usually on the opposite side of the in-feed direction. Moreover, to separate the PCBN grains from the macro-fracture during grinding, the uncut chip thickness per grain should be kept smaller than 1.0?µm to prevent the unfavourable fracture behaviour from appearing around the grain–filler bonding interface. Furthermore, the corresponding single-grain grinding trials are performed to validate the numerical simulation results by evaluating the wear/fracture morphologies of the PCBN superabrasives in the actual grinding operation.     

Effect of composition,quantity, and synthesis temperature of spinels,and some properties of periclase — Spinel compositions

Conclusions The incorporation of spinel (5–10%) into the composition of the solid solution leads to periclase grain growth and to its sintering. Excess spinel in combinations with periclase over this quantity prevents recrystallization sintering of the specimens.A reduction in the synthesis temperature of most the spinels from 1750 to 1450°C contributes the sintering of the compositions under investigation, and improves their technical properties.In the periclase—spinel compositions the maximum influence on the grain growth and the properties of the specimens is exerted by the temperature conditions used for synthesizing the high-alumina spinels, while the conditions used to obtain the highly chromic spinels, and especially the magnesiochromite, have little or no effect on the technical properties of the fired product.With an increase in the spinel phase concentration from 5 to 30%, and also when there is a change in its composition in the direction from magnesia-alumina spinel to magnesiochromite, such factors as strength, porosity, and spalling are impaired.In the MgO-Mg(Al_1–X, Cr_X)₂O₄ system it is possible to obtain strong, dense, and simultaneously spalling-resistant refractories by using small additions of alumina spinel, synthesized at low temperatures ensuring the completion of the spinel-forming reaction.Translated from Ogneupory, No. 3, pp. 53–57, March, 1971.     

Effect of Y2O3 content on densification,microstructure and mechanical properties of reaction sintered magnesium aluminate spinel

Dense magnesium aluminate (MgAl₂O₄) spinels were developed via single-stage solid-state reaction sintering method at 1550–1650^oC using combinations of varied commercial grade reactants-three different sources of alumina and two different sources of magnesia. The effect of Y₂O₃ doping in the concentration range of 1–4 wt % on different spinel batches was studied. Y₂O₃ addition was found to favour the densification of all the spinels at all dopant concentrations and maximum densification was found for the 2 wt % Y₂O₃ containing spinel batches. Phase analysis of the Y₂O₃ containing batches revealed the presence of yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) at all the sintering temperatures. Owing to similar crystal structure isotropic configuration of YAG (cubic) as that of spinel (cubic), Y₂O₃ doping was found to favour densification of spinel. Microstructural investigation revealed that Y₂O₃ containing batches have a controlled grain structure as compared to the without additive batches. Also, 2 wt % Y₂O₃ containing spinel batches sintered at 1650^oC revealed better mechanical properties such as cold modulus of rupture and strength retainment after thermal shock than that of the undoped spinel batches.     

Effect of MoSi2 content on the lubricated sliding-wear resistance of ZrC-MoSi2 composites

The effect of MoSi₂ content (5, 10, and 20 vol.%) on the lubricated, sliding-wear behaviour of ZrC-MoSi₂ composites at room temperature is investigated and modeled. It was found that the resistance to sliding wear decreases markedly with increasing MoSi₂ content, with a greater rate of mild wear, an earlier transition from mild to severe wear, but essentially the same rate of severe wear. The analysis of the wear results using a mechanistic model indicated that the worsened sliding-wear resistance with the increase in MoSi₂ content derives from the decreased hardness and increased internal effective tensile stresses of the ZrC-MoSi₂ composite, which speed up the accumulation of damage induced by plastic deformation within the grains and shorten the onset of the grain-boundary fracture condition and subsequent grain pullout. Reduction of the MoSi₂ content thus emerges as an effective approach for making the ZrC-MoSi₂ composites more sliding-wear resistant under lubrication at room-temperature. These results may have important implications because ZrC holds promise for use in tribological applications requiring both wear resistance and electrical contact, and MoSi₂ is its commonest sintering additive.     

Relationship between fracture toughness characteristics and morphology of sintered Al2O3 ceramics

The influence of sintering temperature and soaking time on fracture toughness of Al₂O₃ ceramics has been investigated. The samples were prepared by solid state sintering at 1500, 1600 and 1700 °C for different soaking time periods. The fracture toughness of the sintered samples was determined by inducing cracks using Vickers indentation technique. Microstructural investigations on fracture surfaces obtained by three point bend test mode were made and correlated with fracture toughness. Crack deflection in the samples sintered at 1500 and 1600 °C for which ranges of fracture toughness are 5.2–5.4 and 5.0–5.6 MPa m^1/2 respectively, are found. The samples sintered at 1700 °C have lower fracture toughness ranging between 4.6 and 5.0 MPa m^1/2. These samples have larger grains and transgranular fracture mode is predominant. The crack deflection has further been revealed by SEM and AFM observations on fracture surface and fracture surface roughness respectively.     

The effect of the inner particle structure on the electronic structure of the nano-crystalline Li-Ti-O spinels

The effect of the inner particle structure on Li insertion activity and electronic structure of the nano-crystalline Li-Ti-O spinels was studied on materials prepared by solid state and solvothermal synthesis. The high temperature prepared materials of composition corresponding to Li₄Ti₅O₁₂ feature particles with characteristic size of ca. 200 nm with randomly distributed defects. The products of solvothermal synthesis with composition Li_1.1Ti_1.9O_4+δ, feature cubic particles of characteristic dimension of ca. 50 nm; the characteristic particle size differs from that of the coherent domain determined by X-ray diffraction. The reduction of the solvothermal and high temperature synthesized nano-crystalline spinels in Li containing solutions leads according to ⁶Li MAS NMR spectra to Li insertion into tetrahedral 8b and octahedral 16c position, respectively. Additional broad NMR signal attributable to a Knight shift was observed in spectra of partially reduced high temperature spinels. In the case of solvothermal spinels is the Knight shift signal less pronounced and appears only in spectra of samples in which the phase transition occurs on the local level. The UV-vis-NIR spectra of the partially reduced Li-Ti-O spinel samples correspond to expected semiconductor character of Li-Ti-O spinels. Both types of materials are characterized by band gap of 3.8 eV (high temperature spinel) and 3.5 eV (solvothermal material). Partial reduction accompanied with Li insertion causes additional optical transition in the visible to near infrared region, which can be attributed to formation of trivalent Ti, character of which changes with degree of reduction. The behavior observed for partially reduced high temperature spinels is similar to that reported for TiO₂ (anatase). The spectral behavior of the partially reduced solvothermal spinels is more complex and reflects suppressed phase transition.     

Preparation of PZT-based piezoceramics with transgranular fracture mode

In this paper, the effect of various doping and substitution elements on the mechanical fracture of perovskite-type (Pb_0.95Sr_0.05)(Zr_0.52Ti_0.48)O₃–Pb(Mn_1/3Sb_2/3)O₃–Pb(Mn_1/3Nb_2/3)O₃ and (Pb_0.95Sr_0.04)(Zr_1?xTi_x)O₃–Pb(Mg_1/3Nb_2/3)O₃ ceramics was investigated. The predominant transgranular fracture mode was observed in the modified piezoceramics mentioned above and the ZnO, SnO₂ and SiO₂ as trace additives were found beneficial to enhance the mechanical bonding force of grain boundaries. The mechanism of transgranular fracture mode was understood by the diffusion of doping elements into the structural lattice of perovskite-type oxides, agreed with the experimental results of reduced tetragonality, dielectric constant and piezoelectric coefficient.     

An indentation model for erosive wear in Al2O3/SiC nanocomposites

The erosive wear resistance of Al₂O₃ has been shown to be improved by the addition of 5 vol.% of sub-micron sized SiC particles to form a ‘nanocomposite’, in agreement with previous results. The erosive wear was measured directly, and also estimated by an indentation model consisting of closely spaced grids of indentations that mimic the effect of successive particle impacts; in the model, particle impacts cause sub-surface cracking but loss of material from the surface occurs only from an impact within a region damaged by a previous impact. The volume of material lost from within indentation grids was used to predict the wear rate. These predictions agreed well with the directly measured values.The commonly observed change in fracture mode from intergranular for Al₂O₃ to transgranular for nanocomposites was confirmed. Transgranular fracture can allow a smaller volume of material to be removed during an impact and hence increase erosion resistance.     

Degradation of TiB2 ceramics in liquid aluminum

Infiltration of liquid aluminum in three TiB₂ materials with significantly different microstructures at 1000 °C has been studied. The results show large differences in resistance towards aluminum penetration. A TiB₂ material containing transition metal sintering aid, displayed a low resistance towards penetration of liquid aluminum along grain boundaries. Grain boundary penetration was also observed in a second material containing an oxycarbide secondary phase TiC_1−xO_x, while Al infiltration could not be detected after 100 h, in a third material with apparently no secondary phases. The flexural strength, hardness and stiffness of the exposed material showing the lowest resistance towards penetration of aluminum, decreased due to penetration along grain boundaries, and the fracture mode changed from transgranular to intergranular. The experimental observations were analyzed using a two-dimensional finite element model, reproducing the observed reduction in the stiffness due to aluminum infiltration.     

Direct additive manufacturing of melt growth Al2O3-ZrO2 functionally graded ceramics by laser directed energy deposition

Functionally graded ceramics (FGC), which combine properties of different ceramics in one part, usually have better comprehensive function and structural efficiency. In this study, four different gradient transition Al₂O₃-ZrO₂ FGC samples were prepared by laser directed energy deposition (LDED) method. The results show that there is an obvious interface in direct transition sample. The transition section bears tensile stress caused by difference of thermophysical properties of materials, resulting in significant longitudinal cracks. Element transition in interface region shows a step sharp transition. The direct transition sample shows intergranular fracture and the bonding strength is very low. Gradient transition mode can effectively suppress cracks, and avoid the step transition of microstructure and elements. Elements, microhardness of 25, 20 wt% FGC samples realized a nearly linear smooth transition. The interface fracture of FGC samples changed to transgranular fracture, bonding strength was significantly improved, and the maximum flexural strength reached 160.19 MPa.     

Effect of reaction of periclase and iron-chrome spinel on the properties of periclase-spinel refractories

Conclusions Using a typical composition of the type MgO-Mg(Al_1–x–yCr_xFe_y)₂O₄ a study was made of the effect of the processes involved in the processing and decomposition of solid solutions of periclase and spinels on the technical properties of the fired products.During cooling from 1600–1750°C decomposition occurs in the solid solutions of periclase and spinel; the crystals of spinel separating under these conditions are able to grow to large sizes which inevitably should lead to the development of stresses inside the periclase grains.The decomposition of the solid solution of periclase and spinel adversely affects the strength and thermal-shock resistance of the fired specimens.The best technical properties are possessed by combinations of periclase with small (up to 15%) quantities of high-alumina (x=0.16 and 0.46) spinels.The use of compositions of periclase with high-alumina spinels of the type x=0.16 and 0.46 enables us to obtain denser and more thermal-shock resistant refractories.Translated from Ogneupory, No.6, pp. 44–47, June, 1970.