# Coupled Temperature–Oil/Water Ratio Effects on Tribo-Chemical Reactions and Failure Behavior of Polycrystalline Diamond

## TL;DR

This study examines how temperature and oil-water ratios affect the wear and friction of polycrystalline diamond in drilling tools.

## Contribution

The novel contribution is identifying how oil-water ratios and temperature jointly influence tribo-chemical reactions and failure mechanisms in PCD.

## Key findings

- High oil-water ratios increase friction and wear with rising temperature due to thermal stress and poor transfer film formation.
- Low oil-water ratios show non-monotonic friction behavior linked to surface passivation and water evaporation effects.
- Temperature transitions near 100 °C mark lubrication regime shifts in water-rich drilling fluid environments.

## Abstract

Polycrystalline diamond (PCD) compacts are extensively applied in downhole drilling tools owing to their exceptional hardness and wear resistance. However, their tribological performance is strongly influenced by the thermal and chemical characteristics of drilling fluids. In this study, the coupled effects of temperature (25–125 °C) and oil–water ratio on the tribological behavior of PCD were systematically investigated. The results indicate that under relatively high oil–water ratios (50:50, 80:20, and 100:0), both the friction coefficient and wear rate increase monotonically with temperature, which is associated with intensified interfacial thermal stress and suppressed formation of protective carbon-based transfer films. In contrast, at low oil–water ratios (0:100 and 20:80), the friction coefficient exhibits a non-monotonic dependence on temperature, decreasing initially and then increasing with a transition near 100 °C. This behavior is attributed to temperature-activated surface passivation through C-OH bond formation in water-rich environments, followed by the deterioration of passivation due to water evaporation at elevated temperatures. These findings provide insight into temperature-dependent lubrication regime transitions and tribo-chemical evolution of PCD in complex drilling fluid environments.