Key Engineering Materials Vols. 336-338

Abstract: To study the deformation reaction of different metallurgical phases of the micron structure of crack tips of zirconia ceramics in the loading state, the paper presents the micro experimental study based on the digital correlated method of analysis by scanning electron microscope on the displacement field in the micro areas of crack tip of the specimen of the Y2O3-ZrO2 toughened ceramic with the simple edge notch beam method and under the action of three-point bending loads. First, SEM analyses was made on the local area of the crack tip of a specimen’s polished surface before and after loads are applied to the specimen, thus obtaining two SEM images, one before the application of the loads and another after the load application, which contained their displacement information. Then, digital correlation calculations were made between the gray scales of the pixels of the two images in order to obtain the microscopic displacement fields of the crack tips in the microstate, thus unveiling the deformation modality and displacement response of different metallurgical phases of the crack tips of zirconia ceramics when subject to loads. The results of the studies will provide foundations and basis for further research in the mechanical properties of the ceramics with Y2O3-ZrO2 and their relationship with the microstructures.

Abstract: In this paper, the lower and upper bounds of Young’s modulus and Poisson ratio are determined by the effective stress method and effective strain method from material mechanics. The effective heat flux method based on the hypothesis of equivalent temperature difference and the effective temperature difference method based on the hypothesis of equivalent heat flux are proposed, the lower and upper bounds of the thermal conductivity of ceramic/metal composite material are determined similarly. The result shows that the property values of ceramic/metal composites calculated from current theoretical property formulas are within the lower and upper bounds determined by the above methods.

Abstract: The pressure sensitive transformation criterion is used to determine the constitutive relation of transformation toughened ceramics. By the use of the strain energy release rate criterion and the method of weight function, the fracture enhancement of mixed-mode I-II crack in transformation toughened ceramics is predicated. The theoretical expressions of the toughening effect for both the stationary and steady-state growing crack are given respectively. The result show there is no toughening effect for the stationary crack and the toughening effect for the steady-state growing crack is associated with the modulus of elasticity, the width of transformation and its volume fraction.

Abstract: The low thermal expansion (α25-1100oC = 0.05 ~ 1.6 × 10–6/K) of Al2TiO5 ceramics are apparently due to a combination of grain boundary micro cracking caused by the large thermal expansion anisotropy of the crystal axes of the Al2TiO5 phase. During the reheating run, the individual crystallites expanded at low temperature; thus, the solid volume of the specimen expanded into the micro cracks, where as the macroscopic dimensions remained almost unchanged. As a result, the material expanded very little. The micro cracks closed at higher temperatures. This result is closely related to relatively steeper thermal expansion curves.

Abstract: The ceramic coating was formed by micro-plasma arc oxidation (MPAO) on AZ91D magnesium alloy. The surface and section morphology of coatings were observed using scanning electron microscopy. The phase composition of coatings was analyzed by X-ray diffraction. The method of salt fog experimental was carried out to proof-test performances of anti-corrosion of material. The results showed that the ceramic coating was composed by loose layer and compact layer. The coating surface has a large number of grains with various sizes. In addition, there is also a lot of pore in the loose layer, but the compact layer is tighter than the loose layer. Compact layer has a good combination with substrate magnesium alloy. The MPAO coating is mainly composed of silica oxide (MgAl2Si3O12 and β-Mg2SiO4 and (Mg4Al14) (Al4Si2)O20) and composite oxide of Mg and Al (δ-MgAl28O4). The performance of resistant corrosion of AZ91D coved by ceramic coating is higher than AZ91D magnesium alloy. The corrosion ratio of AZ91D alloy coved by ceramic coatings to AZ91D alloy is 1:8.61.

Abstract: Duty ratio is a key factor during micro-plasma oxidation process. Its influences on structure and corrosion resistance of ceramic coatings on Ti–6Al–4V by pulsed bi-polar MPO in NaAlO2 solution were investigated. The ceramic coatings were composed of Al2TiO5, α-Al2O3 and rutile TiO2, of which Al2TiO5 was the main crystalline. The coating can be divided into a porous outer layer and a dense inner layer. Compared with the condition of the same duty ratio for both pulses, the rise of anode duty ratio (D1) led to an increase in the amount of Al2TiO5 and α-Al2O3 and increased the thickness of the coating; whereas the rise of cathode duty ratio (D2) led to a decrease in the amount of α-Al2O3 and decreased the thickness of the coating. Besides, the corrosion resistance of the coated samples was better than that of Ti-6Al-4V substrate. When D1 / D2 = 60 / 30, the produced coating’s corrosion resistance was best.

Abstract: Porous Al2O3 ceramics were produced by a dry pressing technique and their characteristics were assessed. In terms of their microstructure the results showed that the open porosity varies with the type of pore-making agents. The open porosity is increased but fracture strength is decreased with an increase in the amount of pore-making agent. The fracture strength of Al2O3 ceramic matrix is increased by adding 5-15wt% ZrO2 particles (ZTA). In terms of their wet erosive wear characteristics the results from liquid-solid impinging jet tests can be used to determine the weak aspects of their microstructure. From examination of worn surfaces it is apparent that grain pull-out is the dominant mechanism of erosive wear. For the porous ceramics, wet erosive wear rate depends on strength, porosity and pore size. For porous Al2O3 ceramics having the same level of porosity, the larger the biggest pore size the higher the erosive wear rate. ZTA composite ceramic has a higher erosive wear resistance than that of Al2O3 ceramic with an equivalent level of porosity primarily due to its higher strength.

Abstract: A slurry pot tester was used to evaluate the erosion behavior of alumina (Al2O3), zirconia toughened alumina (ZTA), and silicon nitride ceramics. The eroded surface of sample was studied by scanning electronic microscope and the primary erosion wear mechanisms for ceramic materials were analyzed. On the same experimental condition, erosion wear resistance of materials follows this rule: Si3N4>ZTA>Al2O3. The key factor affecting erosive wear performance of ceramics is their microstructure. The primary erosive wear mechanisms for Al2O3, ZTA and Si3N4 ceramics are lateral crack propagation, whole grains pulled out and plastic deformation. The erosive wear performance of Si3N4 ceramic is the best in this experiment, due to their microstructure morphology with elongated grains.

Abstract: Erosion wearing regulation with different erosion angel for Si3N4 ceramics were systematically studied by liquid-solid dual phase erosion wearing test with rotation disk through comparing with Cr15Mo3 high chromium cast iron for pumps in industry. Besides the micro failure mechanism and the relationship between mechanical behavior and anti erosion wearing have been analyzed for the experimental materials. The experimental results were concluded as following: the erosion resistance of Si3N4 ceramic is about 20 times better than that of Cr15Mo3 high-Cr cast ion in erosion test angle, and the effect of erosion angel on the erosion wearing volume lose features as N-shaped curve with the biggest volume lose at 45° erosion angle while most erosion wearing volume at 90° angle, which indicates that it should avoid 45° or 90° erosion angle during materials designing. Obvious direction and erosion trace are revealed and volume lose mainly featured with tiny cutting, pear ditch and erosion hole, and micro crack and grain falling off at high angle erosion on the surface of Cr15Mo3 materials, besides smooth surface, high ductile, compact structure, fine grain and fine cylinder grain explaining main reason for excellent anti erosion wearing for Si3N4 ceramic.

Abstract: The wear behaviours of alumina strengthen 3Y-TZP ceramics worn against high chromium cast iron under various testing conditions were investigated using a block-on-block tribometer. The results showed that the wear rate increases with the increasing of the load and the sliding time. Under the low load the main mechanism governing the wear properties of ceramic is the apparent plastic deformation and ploughing, and the wear rate rapidly increases and the wear mechanism changes into brittle fracture under high load. Meanwhile the wear resistance is depended on the properties of the abrasive particle. As expected, the wear resistance was increased with decreasing hardness of abrasive particle, but it was also influenced by the morphology, that is, the sharper the abrasive particle the higher wear rate.