tailieunhanh - Corrosion of Ceramic and Composite Materials Part 13

Tham khảo tài liệu 'corrosion of ceramic and composite materials part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Properties and Corrosion 349 tests indicated the presence of stress-enhanced oxidation at 1000 C with failure times ranging from 19 to 93 hr at an applied load of 138 MPa and from 14 to 31 hr at an applied load of 276 MPa. Losses in strength at temperatures greater than 1200 C were attributed to the softening of the glassy grain boundary phase which leads to creep by grain boundary sliding. Samples exposed to oxidation at 1200 C at an applied load of 344 MPa did not fail even after 260 hr although some slight deformation had occurred. In an effort to determine the effects of oxidation upon the flexural strength of Si3N4 Kim and Moorhead evaluated the room-temperature four-point bend strength of HIP-SN with 6 wt. Y2O3 and wt. Al2O3 after exposure in either H2 H2O or Ar O2 at 1400 C for 10 hr. In both atmospheres the strength was dependent on the amount of oxidant present. However the actual variation in strength was different depending upon the alteration of the surface layers formed and their characteristics. In the H2 H2O atmosphere at low pH2O a nonprotective and not well-attached glass-like layer containing crystalline Y2Si2O7 formed. Because this layer was relatively uniform with no new strength-limiting flaws being formed although some large bubbles were found at the surface substrate interface the maximum reduction in strength was limited to about 20 at a pH2O of 2x10-5 MPa. A significant strength increase occurred as the pH2O was increased which the authors attributed to blunting of preexisting cracks by the interfacial silicate phase. This silicate phase was a continuous dense layer of Y2Si2O7 containing small isolated bubbles believed to be formed by nitrogen generation during oxidation of the Si3N4. In the Ar O2 atmosphere a similar reduction and subsequent increase in strength was not found. Instead at low pO2 an increase in strength occurred with increasing pO2. The maximum strength occurred at pO2 10-5 MPa that yielded the greatest weight .