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Are bolts and screw threads useful for ceramic joining?


Frequently asked questions

Ceramic components can be bolted into place, which usually requires a hole in the ceramic. This can be produced either during manufacture or by machining afterwards.

If accurately sized holes are required, diamond machining may be necessary but this will significantly increase production time and costs. Machining holes into a green-state (pre-fired) ceramic is relatively easy, but on firing, dimensional control is more difficult. In either case it is beneficial to ensure that the hole (or feature) does not have microcracks at the surface; these would detract from the fatigue performance.

An alternative to bolting is to machine a screw thread on to the ceramic.

Screw threads may be used in cases where the load on the thread is not high. One of the best examples is a ceramic welding nozzle. Here the thread acts to hold the ceramic and metal together in position while the interface carries most of the load.

High contact loads and high stress concentrations in the thread roots can lead to problems, particularly if the joint is over tightened. Thus extreme care should be taken in attempting to use threads in design and best practice (correct torque) must be employed.

Even simple threaded designs may require complex diamond machining, which makes such joints expensive; moreover, they may not be gas tight. Care must also be taken to ensure that machined features such as screw threads do not become stress concentrators.

Ceramics are particularly prone to cracking at notches and severe changes in section. In addition, some machining operations can induce microcracking at the surface, again leading to premature failure.

To overcome these difficulties, a range of ceramics has been microstructurally engineered specifically to be machinable. This is achieved by incorporation of phases or grains (such as mica or hexagonal boron nitride) with high aspect ratios. These then act in the same manner as sulphide inclusions in free machining steels.

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