Papers by Author: Zheng Fang Yang

Abstract: The wear properties of ADZ (alumina dispersed in Y-TZP) and MDZ (mullite dispersed in Y-TZP) were investigated by using a ring-on-block tribometer. The results showed that for Y-TZP ceramic, the addition of alumina phase (with 10-20% in mass fraction) leads to an improved wear resistance. With the increase of the normal load, the wear rates of ADZ ceramics increase. Under low and medium normal load (100N and 300N), the wear resistance is controlled by the hardness of ceramics, and under high normal load (500N) the fracture toughness is obviously contributed to the wear resistance of the ceramics. For MDZ ceramic, the wear resistance of 15MDZ (15wt% mullite dispersed in Y-TZP) is better than that of 20 MDZ (20wt% mullite) under the normal load from 100 N to 500 N. The mechanical properties of 15MDZ are worse than that of Y-TZP ceramic, but the wear resistance is enhanced due to the action of “needle roller bearing” of the fractured rod-like mullite particles.

Abstract: The correlation between machining performance and mechanical properties of Y-TZP/Al2O3 ceramics with different mechanical parameters was investigated. The grinding experiments indicated that the flexural strength had a definite influence on machining performance of samples. The higher the flexural strength was, the more difficult the sample was to be ground. XRD analysis showed that an increasing amount of the monoclinic ZrO2 would reduce the flexural strength of the samples. As a result, the grinding force that the samples underwent also decreased.

Abstract: To study the effect of second sintering on intragranular structure and mechanical properties of 10vol%Al2O3/3Y-TZP composites, the composites were prepared by ball-milling nano-γ-Al2O3 and nano-ZrO2(3mol%Y2O3), pressing unidirectionally, cold-pressing isostatically, then first sintering at 1400°C for 2h and second sintering at 1600°C for 5h in air. The composition and microstructure of the composites were examined by X-ray diffractometer and scanning electron microscope, transmission electron microscope. The results showed that the second sintering changed the microstructure and fracture mode. After second sintering, the structure of fine grains was changed into the mixed one of large grains and small grains . Many large grains contained small grains, forming intragranular stucture that could be produced by the interblending of not only Al2O3 and ZrO2 but also ZrO2 itself. The intergranular fracture after first sintering was changed into transgranular fracture and intergranular fracture after second sintering . The second sintering led to grinds growing up and intragranular forming, which enhanced the contribution of the phase transformation toughening. Although the second sintering resulted in abnormal increase of grain size, the fracture strength and the fracture toughness of the composite were 667 MPa and 12.9 MPa⋅m1/2, respectively, due to the intragranular structure and the enhanced phase transformation toughening.

Abstract: Cubic boron nitride(CBN) is a superhard materials with many advantages and many uses. Vitrified bond CBN grinding tool is a promising abrasive tool of high performance used for high speed, high efficiency, high precision grinding with lower grinding cost and less environment pollution. Sintering of vitrified bond CBN grinding tool was investigated in this paper. The results showed that practical sintering temperature of this tool was much lower than the initial oxidation temperature of CBN particle measured by comprehensive thermal analysis. The upper limit of sintering temperature should be determined by taking account of the thermal analyzing results, heating process of CBN and its change in strength and structure. Within the sintering temperature range of the vitrified bond, relatively higher sintering temperature was beneficial to the strength of bond bridge and the holding strength between bond and CBN abrasive particles. CBN tool sintered at relatively lower temperature tended to fracture through the bond bridge, while the one sintered at higher temperature tended to fracture along the boundary between CBN abrasive grain and vitrified bond.

Abstract: The effect of microstructure on the wear resistance of two Al2O3 dispersed in ZrO2 matrix (ADZ) ceramics was investigated, Al2O3 dispersed in commercially co-precipitated 3mol% Y2O3-doped tetragonal zirconia polycrystal (CYADZ) and Al2O3 dispersed in mixed 3mol% Y2O3-doped tetragonal zirconia polycrystal (MYADZ). These materials were used as a pin sliding against a plate of polyurethane rubber (UR) in the presence of aqueous NaOH solution containing SiO2 as the abrasive medium at a sliding speed of 0.15m/s and a normal load of 100N. The results indicated that the CYADZ specimen had uniform microstructure and fine grain size, which accounted for its better abrasive wear-resistance than that of the MYADZ specimen with non-uniform microstructure and largely different grain sizes due to inhomogeneous distribution of Y2O3. The hardness and toughness of two ADZ specimens were close, but strength of the MYADZ specimen was lower than that of the CYADZ specimen, which indicated the defect was serious in the MYADZspecimen. The CYADZ specimen was characterized by plastic deformation and micro-ploughing, which the MYADZ specimen dominated by micro- cracking and brittle fracture.