Dry lubrication of ferritic stainless steel functionalised with crystalline aggregates of hexadecylphosphonic acid

Highlights

• Crystalline platelets of hexadecylphosphonic acid are physisorbed on the surface through a new functionalisation method.

• A tribofilm is established on the friction track through the shearing of the crystalline platelets.

• The tribofilm confers low-friction and more stable behaviour compared to the self-assembled monolayer alone.

• The friction coefficient decreases as the contact pressure increases.

Abstract

This paper demonstrates the excellent tribological behaviour afforded by a new functionalisation technique based on the combination of chemisorption and physisorption of self-assembled hexadecylphosphonic acid for application in metal forming. Surface functionalisation of an X2TiCrNb18 ferritic stainless steel with this technique led to the growth of aggregates on the surface. Detailed analysis showed that crystalline aggregates were physisorbed on a chemisorbed molecular layer. Ball-on-plate linear reciprocating sliding tests were performed under high maximal contact pressures in the 0.5–1.1 GPa range using a 100Cr6 counterpart. Results revealed exceptional low-friction and low-wear afforded by a tribofilm originating from aggregates shearing. The tribofilm built up very quickly and presented the interesting feature of reducing the friction coefficient as the contact pressure increased.

Introduction

Stainless steels are metallic alloys widely used in numerous everyday-life and industrial applications. Among them, a large amount of sheet or shell shaped components are formed through stamping, embossing or rolling operations. Since high mechanical stresses are transmitted to the tool/material interface to reach the required plastic deformation to form stainless steel products, lubricants are necessary for two main reasons. First: to ensure surface integrity of both the tool and the component: this issue is of major concern regarding stainless steels forming operations since well-known adhesion phenomena easily occur in the absence of lubricant [[1], [2], [3]]. Second: to minimize friction forces at the tool/material interface, in the perspective of costs and energy consumption reduction.

For many years, mineral-based lubricants containing friction modifiers and extreme pressure additives have been widely employed in metal forming and they are still known to be the most efficient ones. Indeed, these compounds tribochemically react with the metal surface to form highly efficient tribofilms whose shearing ensures the accommodation of the surfaces [4]. However, lubricant additives are chlorine, sulphur and/or phosphorous compounds that are nowadays facing to environmental restrictions due their toxicity and poor biodegradability. The development of efficient “green-lubricants” dedicated to metal forming is thus a major challenge. “Bio-lubricants” based on vegetable oils aroused interest in the last years but their potential use in metal forming is confronted to their low oxidative stability, poor low-temperature rheological behaviour, and the necessity to incorporate nanoparticles of metal oxides to improve their efficiency [[5], [6], [7]]. Moreover and from a wider prospective, the use of lubricants in liquid form infers an additional issue since they have to be eliminated after metal forming operations in most cases. Subsequent cleaning operations have thus to be performed to complete further manufacturing operations (welding, soldering, polishing …), leading to time-consumption and additional costs.

Functionalisation of metal sheets with organic molecular species offers a convincing option to circumvent the above-mentioned drawbacks of classical lubricants. This leads to the chemisorption on the metal surface of an organised molecular layer, so-called self-assembled monolayer (SAM), through the simple immersion of the metal into a weakly concentrated solution of reactive molecules dissolved in a solvent. Grafting of SAM has proven to be a powerful method to control surface properties, including frictional ones [8], due to its simplicity, adaptability, and reproducibility [9]. Improvement of the tribological behaviour of various metallic surfaces was reported by grafting highly studied SAMs such as organothiols or organosilanes [[10], [11], [12]]. Notwithstanding, alkylphosphonic acids have naturally aroused much more interest in the past few years due to their lower toxicity and their ability to form strong O–P bonds on surface metal oxides [13]. Grafting of alkylphosphonic acids and subsequent improvement of tribological properties were successfully achieved on copper [[14], [15], [16]], aluminium oxides [17,18], titanium alloys [19], Ti incorporated carbon coatings [20], TiO₂-coated galvanized steel [21], and stainless steel [22].

However, almost all studies were focused on the behaviour of well-organised SAMs whose realisation implies complex surface preparation and the removal through ultrasonication of physisorbed species that were not chemically adsorbed on the surface. Such additional treatments are hardly achievable at the industrial scale in the framework of metal forming. A few years ago, the authors reported exceptional low-friction and low-wear properties exhibited by dodecylphosphonic acids grafted on copper, due to the presence of physisorbed molecular clusters on the surface [16]. This finding led to a new functionalisation technique avoiding surface post-treatment and deliberately promoting the existence of physisorbed molecular aggregates on the original chemisorbed monolayer. Recently, the new functionalisation route was patented [23] and applied in stamping operations realised on stainless steel in industrial framework [24]. The performance of a lubricant for stamping operations, in terms of drawability, is usually quantified by the Limiting Drawing Ratio (LDR) defined as the ratio of the maximum blank diameter to the punch diameter that can be drawn successfully without failure [25]. The drawability increases with increasing LDR values. Whereas oils traditionally used in metal forming (chlorinated or not) leads to LDR ranging from 1.98 to 2.25, the new functionalisation technique was found to achieve LDR ranging from 2.24 to 2.35²⁴. This lubricating method therefore has the double advantage to provide similar or even better performance than the best conventional oils while avoiding cleaning operations. Nonetheless, both the characterization of this advanced functionalisation and the tribological mechanisms involved have not been published yet.

The present paper consequently addresses those limitations. This study focuses on the tribological behaviour of hexadecylphosphonic acids adsorbed on a ferritic stainless steel. The growing of crystalline aggregates originating from clustered hexadecylphosphonic acids is demonstrated and characterised for the first time. A fully instrumented tribometer is used to perform tests in relevant contact pressures with respect to metal forming operations. The tribological behaviour afforded by the new functionalisation technique is compared with this of stainless steel substrate functionalised (or not) with a classical monolayer of the same molecule. An approach coupling friction force measurements and surface modification characterization is conducted. It reveals improved tribological properties that are induced by the presence of crystalline aggregates, building up and then destruction of a protecting tribofilm, and the transient frictional behaviour of this film.