In-Situ Linear Reciprocating High Pressure Tribometry
In-Situ tribological testing of Acrylonitrile Butadiene Rubber (NBR) elastomers under realistic High pressure gas environments is demonstrated in this case study. The equipment used is a Linear Reciprocating Tribometer housed in an autoclave. Hydrogen effects on NBR are apparent from increased friction and wear, compared to high pressure Argon or ambient air.
The tribological behaviour of contacting elastomer and metal surfaces in high pressure gas applications depends upon the type of gas and pressure temperature envelopes. Dynamic loads on the materials, due to pressure and thermal cycling need consideration. Viscoelastic behaviour of elastomers affects the coefficient of friction and elastomer wear-rate increases by adhesion to the metal surface. Hydrogen induced degradation of elastomeric materials is a concern for elastomers deployed in Hydrogen infrastructure. Tribological testing in high pressure Hydrogen environments, simulating actual operating conditions, provides realistic performance data for elastomeric components in high pressure Hydrogen service.
In-Situ tribometers enclose the test samples, tribometry equipment and sensors within a pressure vessel, mitigating safety risks due to high pressure hydrogen. In-Situ Linear Reciprocating tribometry enables high pressure tribological tests of linearly sliding elastomer- metal surfaces in a Hydrogen environment.
Acrylonitrile Butadiene Rubber (NBR) is used for many dynamic sealing applications in high pressure Hydrogen dispensing systems. Typical applications include NBR seals for Hydrogen compressors, seals and stem packings in isolation and control valves, diaphragms and actuators. NBR elastomers are known to degrade in Hydrogen environments due to gas permeation and dissolution. In dynamic sealing applications of NBR, wear and tear present additional concerns.
This case study uses a bespoke pin on flat type linear reciprocating tribometer system to investigate the tribological performance of NBR in high pressure Hydrogen, Argon and Air environments. Controlled frictional loading is provided by a ball at the tip of a steel pin pressing upon an NBR sample moving in a direction perpendicular to the loading force, at various normal loads, velocities and frequencies. In this case, a disk-shaped sample of NBR is clamped horizontally and the weight loaded ball runs linearly, back and forth, across the sample surface to form a wear track. Figure 1 (A) illustrates the principle, while Figures (B) and (C) are a photograph of the apparatus.
NBR Samples of 1/8th inches (3.175 mm) thickness with 40A Shore hardness rating were cleaned and subjected to tribological tests in ambient air at atmospheric pressure, Argon at 24 MPa and Hydrogen at 28 MPa. Wear measurements from the in-situ linear variable differential transformer (LVDT) were confirmed by ex-situ measurements done subsequently, using optical profilometry. It was observed that sample wear tracks in the high-pressure Hydrogen tests showed more pitting, depressions, and cracking, than those from High pressure Argon testing. Additionally, the coefficient of friction was maximum for the Hydrogen exposed samples.
Figure 1. (A) Illustration shows apparatus and working principle. (B) Photo of Linear Reciprocating Tribometer apparatus with key components labelled. (C) The Tribometer which is approximately 7 inches high and 5 inches across, being inserted into the pressure vessel (autoclave).