it may provide for a change in the friction forces either by more or less friction which can be measured or would otherwise be detectable by an operator of the machine or be measured automatically or it can cause an alteration in the processed fiber which is detectable as being characteristic of the wear on the part in question by way of example of the invention but in no way intended to be limiting the invention applied to the coating of a combing roll of the type used in open end textile spinning machines it should be understood that the invention is not limited to an apparatus with only an indicator layer and a wear resistant layer in practice the novel apparatus may also include other layers either under and or over the wear resistant layer and or indicator layer typically suitable wear resistant layers include nitride carbide or oxide layers particularly those of the refractory metals such as titanium hafnium and tungsten or those of aluminum silicon and boron metallic layers such as chromium or nickel or alloys thereof and composite layers comprising a metal such as chromium or nickel having small wear resistant particles codeposited therewith these particles typically can include metallic oxides carbides or nitrides diamonds or lubricating particles such as teflon graphite fluoride particles and the like the methods for depositing coatings of the types set forth above are well known in the art the indicator layer may be selected from a variety of materials as long as the indicator layer is capable of indicating that the functional layer has eroded for example the indicator layer may be a copper layer plated on the substrate such that when the composite layer has worn through.