Plastic and nonmetallic article shaping or treating: processes – With severing – removing material from preform mechanically,... – Surface finishing
Patent
1990-05-07
1992-07-07
Lowe, James
Plastic and nonmetallic article shaping or treating: processes
With severing, removing material from preform mechanically,...
Surface finishing
264293, B28B 1108
Patent
active
051280833
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for modifying the surface of hard engineering ceramic materials. These materials include compounds (oxides, carbides, nitrides, borides) of the elements silicon, boron and other transition metals, and for the purposes of the present invention can be generally defined as having a room temperature Knoop indentation hardness, using a 1 kg normal load, greater than 1000 kg/mm.sup.2 (or 10 GPa).
Hard engineering ceramic materials are relatively new and in many cases the full extent of their useful applications is yet to be fully explored. However, if is known that such materials have a tendency to fracture, rather than to deform plastically, when subjected to stress at temperatures below about 0.5 of their melting points (0.5 Tm)--that is to say, they tend to be brittle at such temperatures. Consequently they are difficult to shape and, most often, components made from such materials must be made by hot pressing or sintering processes followed by grinding to the final dimensions. Grinding inevitably introduces micro surface defects and cracks which can contribute to premature failure whilst in service, particularly in tribological applications such as a piston within a piston cylinder. These difficulties in working such materials obviously limit their useful applications.
It is an object of the present invention to provide a process for modifying the surface of hard engineering ceramic materials and, more specifically, a process whereby micro surface defects and cracks in the surface of the material can be annealed and/or sealed.
According to the present invention there is provided a process for modifying the surface of hard engineering ceramics materials wherein the surface of the material is subjected to plastic deformation caused by point/line loading transmitted by the surface of a second material.
Throughout this specification the term "point loading" is intended to define forces transmitted from a pointed surface to a plane surface as between a pencil and paper. The term "line loading" is intended to define the forces transmitted from a line surface to a plane surface as between a wedge, chisel or cylinder and a plane surface.
Preferably, the second material is loaded against the hard engineering ceramics material which is being treated at a sufficiently high temperature to cause the second material to flow into and replicate the surface features of the hard engineering ceramics material by creep. Conveniently, this process is carried out at a temperature less than 0.5 Tm. (Tm equals the temperature at which the hard engineering ceramics material melts). The second material may conveniently comprise a second hard engineering ceramic material, but this may be of the same hardness or even softer than the hard engineering ceramic material which is being treated.
Using the process of the present invention two major benefits accure:
1. Firstly, defects introduced by grinding the material, such as cracks, are healed with a consequent improvement in wear resistance.
2. Secondly, small changes in surface topography can be developed and controlled. Thus, one ceramic surface can be caused to develop optimum conformity to another by deforming these surfaces in a non-impactive manner under creep conditions. This means that a piston and piston cylinder can be treated to conform with each other for optimum tribological coupling in use at normal operating temperatures.
Conveniently, the surface of the hard engineering ceramics material can be hardened by repeatedly traversing (rubbing) it with a softer lubricated metal. It has been shown by experiment that the hardness of the hard engineering ceramics material may be increased by some 60% in this way and the increase in hardness is permanent. Furthermore, the wear resistance of the material is increased under three-body-wear conditions that is to say where wear debris or oxide particles are trapped between two surfaces which are wearing against one another.
It has been found that the principle variables which have to be control
REFERENCES:
patent: 4168183 (1979-09-01), Greenfield et al.
Fiorilla Christopher A.
J. H. Fenner & Co. Limited
Lowe James
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