Comparative study of tribofilm growth on steel surfaces in the presence of HFO and HFC refrigerants

Wasim Akram1*, William Lapp2, Mike Benco3, Donna Bossman4


1. Materials Engineer, Ingersoll Rand, 314 W 90th St, Bloomington, MN

2. Sr. Materials Engineer, Ingersoll Rand, 2313 S 20th St, La Crosse, WI

3. Compressor Engineer, Ingersoll Rand, 2313 S 20th St, La Crosse, WI

4. Director, Network of Excellence Materials & Chemistry, Ingersoll Rand, 2313 S 20th St, La Crosse, WI

* Corresponding author: wasim.akram@irco.com

Abstract

Developing in-operando tribofilm on iron-based surfaces is one of the key characteristics of environmentally friendly HFO-based refrigerants. Previous studies were focused on cast iron interfaces in the presence of HFO-1234yf refrigerant. As a continuation of past work, we studied comparative wear mechanisms of 52100 steel surface in the presence of several other widely-used HVAC refrigerants, namely HFO-1234ze(E) and HFC-134a.  Herein, we conducted controlled tribological experiments to simulate refrigerant-compressor specific bearing contact conditions under R1234ze and state-of-the-art R134a refrigerants. These experiments demonstrated superior wear and friction characteristics in the case of HFO-1234ze(E) compared to HFC-134a. The differences in wear behavior were analyzed utilizing Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). These advanced surface chemistry analyses revealed a higher concentration of Fluorine-based species inside the wear track for HFO-1234ze(E) environment compared to HFC-based refrigerants. It is hypothesized that olefin-based refrigerants can release fluorine more readily at the contact zone due to the reactive nature of the refrigerant. Subsequently, released fluorine at the interface can help to develop tribofilms in the case of HFO-1234ze(E) via tribo-chemical interactions. Furthermore, we measured the thickness of the tribofilm using depth profiling. Interestingly, traces of fluorine were obtained within 10 nm of the surface, indicating very thin tribofilms. These results provide insights that can help improve understanding of bearing reliability and lubricant behavior in HVAC applications.