I have had the glue fail on tips and I've seen them fall off during a routine.
I've never used any other probes besides the ruby tipped ones. The only ever had one fail calibration on me I suspect was not due to wear even though it was probably 5 years old or more. Most likely dropped even though no one would admit it.
Ruby
The industry standard and the optimum stylus ball material for a vast majority of measurement applications, ruby is one of the hardest known materials. Synthetic ruby is 99% pure aluminium oxide which is grown into crystals (or "boules") at 2000 °C using the Verneuil process.
The boules are then cut and gradually machined into a highly spherical form. Ruby balls are exceptionally smooth on the surface, have great compressive strength and a high resistance to mechanical corrosion.
Very few applications exist where ruby is not the preferred ball material, however there are two applications where balls manufactured from other materials are recommended.
The first is for heavy duty scanning applications on aluminium. Because the materials attract, a phenomenon known as 'adhesive wear' can occur, which involves build up of aluminium from the surface onto the ball. A better ball material for such applications is silicon nitride.
The second is in heavy duty scanning applications on cast iron. Interaction between the two materials can result in 'abrasive wear' of the ruby ball surface. For such applications, zirconia balls are recommended. Silicon nitride
Silicon nitride possesses many similar properties to ruby. It is a very hard and wear-resistant ceramic which can be machined into high precision spheres. It can also be polished to an extremely smooth surface finish.
Silicon nitride does not have an attraction to aluminium and so does not exhibit the adhesive wear seen with ruby in similar applications. However, silicon nitride does show significant abrasive wear characteristics when scanning on steel surfaces and so its applications are best confined to aluminium.
