Wear Performance Without Lubrication

The monolithic ceramics in Table 1 were run self-mated at 1000 rpm, with temperatures from 290 to 540 C, a diesel exhaust atmosphere, and ring loadings from 1.6 to 7.7 N/mm. The wear rates and friction coefficients were high. The ring specimens wore as much as 0.05 mm in 3 minutes, and the friction coefficient varied from 0.3 to 0.6. Extensive surface cracking and edge chipping occurred on the ring specimens. The Si3N4 and SiC ring specimens experienced large-scale fractures rapidly, while the zirconia ring specimens fractured progressively by chipping from the edges. Figure 2 is an example of the edge chipping and cracking on the edge of the contact surface of a YPSZ ring specimen after dry sliding under the above test conditions. The high friction coefficients, high wear ratés, and extensive cracking indicated that these materials were not satisfactory for service in dry sliding contact for ring/ cylinder appliçations.

Experiments with the plasma-sprayed ring coatings sliding against silicon carbide cylinder specimens also resulted in rapid failures. The tests were run until a dynamic wear monitor indicated that a total of 0.25 mm wear had occurred on the two specimen pairs. Most of the coatings completely spalled off the base metal within 3 minutes of operation. The high friction coefficients (typically to 0.6) probably contributed to the bond failures. The M501, a nickel-base coating, demonstrated the best performance in that it survived 20 minutes of operation. However, the wear of the coatings was 0.15 mm during that time and the friction coefficient ranged from 0.3 to 0.6. Inspection of the surfaces showed that the M501 had transferred a layer to the silicon carbide and was actually running self-mated. These unsuccessful results indicated that operation of the ring/cylinder interface without lubrication was probably not feasible with the monolithic and coating materials considered. Since these materials represent a broad cross section of the general types to be considered, the overall likelihood of successful operation without lubrication appears low.

Table 2. Cylinder and Ring Specimens

Material

Nominal Composition, weight percent

Magnesia Partially Stabilized

3.3 MgO, 3 HfOz,

Zirconia (MPSZ)

bal Zr02

Yttria Partially Stabilized

5.4 Y203, bal ZrOz

Zirconia (YPSZ)

(Z-191)

Alumina Transformation

3.6 Y203, 20 Al203

Toughened Zirconia (ATTZ)

bal Zr02

Sintered Alpha Silicon Carbide

cSiC

Silicon Nitride (NC 132)

2 W, 0.5 Mg,

0 0

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