**1. Introduction**

Grease has widespread application in machinery components such as rolling element bearings, pin bushings and journal bearings [1–7], gears [8–10], slide-ways [11,12], and the like. In these applications, grease composition changes with time, degrading its performance. As a result, the efficiency of the machine deteriorates to the extent that, eventually, the grease can no longer adequately protect the surfaces, at which point failure becomes imminent. To avoid forced shut down, machine operators are required to periodically inspect the health of the grease and replenish or replace it, as deemed necessary.

Grease degradation occurs due to physical changes, chemical changes, or a combination thereof [13–15]. Physical degradation occurs due to bleed-off and/or evaporation of base oil and contamination by particles and/or water. This type of degradation primarily prevails during the shearing of greases below 50 ◦C. On the other hand, chemical degradation is a result of oxidation of the base oil, or depletion of the additives, occurring at temperatures higher than 50 ◦C. In general, grease is more prone to physical degradation at high operating speed (i.e., high shearing rate), while chemical degradation occurs at high operating temperatures or during long-term storage [15]. The focus of the present work is on assessing the degradation of grease due to physical change.

The industry typically measures the physical changes in grease by evaluating its consistency through the worked penetration test provided in the American Society for Testing and Materials (ASTM) standard D217 [16]. In this test, a cone of standard shape and weight is released to fall into a cup of grease, after which the depth of penetration

**Citation:** Khonsari, M.M.; Lijesh, K.P.; Miller, R.A.; Shah, R. Evaluating Grease Degradation through Contact Angle Approach. *Lubricants* **2021**, *9*, 11. https://doi.org/10.3390/ lubricants9010011

Received: 24 December 2020 Accepted: 13 January 2021 Published: 18 January 2021

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into the sample is recorded. The larger the penetration value, the lower is the consistency, and vice versa. ASTM D217, however, requires a large amount of grease, which is not practical when trying to assess the consistency of small amounts of grease taken from roller element bearings or other lube points. To overcome this complication, Rezasoltani and Khonsari [17] employed a rheometer for assessing grease degradation by monitoring the change in the rheological properties and correlating it to the mechanical degradation of the grease. The mechanical degradation was determined by squeezing the grease between two parallel plates and measuring the difference in the plate position after 60 s. The mechanical degradation of the grease can also be determined in a rheometer by measuring the yield stress, zero viscosity, cross-over stress, etc.

Lijesh and Khonsari [18] extended the approach proposed by Rezasoltani and Khonsari [17] for developing a predictive model for determining the degradation of grease from their operating NLGI grade and thereafter estimating their remaining useful life. Specifically, they based their degradation assessment on a relationship between the change in the grease consistency and entropy generation. For example, the drop in the consistency of a pristine grease of NLGI grade 2 to NLGI grade 1 or 0 can be considered an indication of the reduction of performance and the necessity for re-lubrication. Testing via a rheometer requires far less grease compared to D217; however, an expensive rheometer and appropriate technical expertise are required, making it unaffordable for many industries.

Lijesh et al. [19] very recently developed a unique approach to quantify the water repellant properties of grease by measuring the contact angle of a droplet of water on the surface of a grease sample. In this method, a small quantity of grease is spread over a surface, a water droplet is dropped on it, and the contact angle of the water droplet is measured. The contact angle values are dependent on the type and composition of the grease, i.e., thickener, base oil, and additives. Thus, we hypothesize that the degradation of grease can also be effectively characterized using the contact angle approach. The immediate advantage of this approach is that only a small quantity of grease is needed. Ideally, a portable instrument can be built for testing grease performance in the field [20,21].

To validate the hypothesis, experiments were performed by degrading grease in a grease worker and measuring the contact angle after periodic intervals. In the present work, the evaluation was performed for three types of greases. The change in the contact angle values after a different number of strokes was considered for evaluating the degradation characteristics of the grease. To corroborate the findings, the same greases were also tested in a rheometer. To gain further confidence, two of the greases rendering higher and lower variation in contact angle with time were tested for tribological performance in a tribometer.

The outline of this paper is as follows. Section 2 provides the details of the instruments used for shearing grease, i.e., a grease worker for measuring the rheological properties and a rheometer for measuring contact angles. Section 3 is devoted to the presentation of results, followed by a discussion in Section 4. In Section 5, summary and concluding remarks are provided.

#### **2. Materials and Methods**

Table 1 shows the list of eight commercially available greases considered for the present investigation. It includes the base oil, the type of thickener, the color, and NLGI grades of each grease.


**Table 1.** Grease designation with thickener and base oil types.
