These apps can be found in the Google Play store. Searching for "Tribology" does not yield very much, but there are plenty of options from wider search terms such as bearings, lubricants and hydraulics. There is a huge number of free apps to choose from. These are my current favourites. Why not add your own favourites to the list?
Calculator - SKF
General-purpose calculator app with a good unit converter and a lot of powerful features hidden behind a rather strange menu interface. These include analytical graphing, polynomial solver up to 4th order, general scientific equations and links to bearing calculators etc.
Lubricants Viscosity Calc - Flavio Cobucci
Simply calculates the viscosity at any temperature (within the linear region of the Ubbelohde diagram...) given the 40 and 100 degrees C readings. Like many people I set this up in a spreadsheet, but I don't often carry my PC around with me.
O-ring - Trelleborg Sealing Solutions
This is an o-ring selector based on ISO3601. I've used this in the office as a design tool, and it seems to work well. The fluid/material compatibility guideline is great.