Wear resistance of CVD and PVD multilayer coatings when dry cutting fiber reinforced polymers (FRP)

Highlights

• We address the wear resistance of multilayer coating when cutting FRP composites.

• We deduce apparent friction behavior and correlate it with material removal process.

• We examine worn face and characterize wear mode versus coating type and fiber type.

• We explain the tribological signature on coating top-layer using fiber-scale analyses.

Abstract

This work discusses the performance of multilayer coatings in dry cutting fiber reinforced polymers. The cutting tests were performed on unidirectional carbon/epoxy and glass/epoxy specimens with 45° fiber orientation using both Chemical Vapor Deposited (CVD) and Physical Vapor Deposited (PVD) multilayer coatings with neatly different composition, grain size and substrate-to-coating adherence. CVD TiCN/Al₂O₃/TiN with low adherence (CVD-L), PVD TiAlN/AlCrO with medium adherence (PVD-M) and PVD TiN/TiAlN with high adherence (PVD-H) inserts were considered for experimental tests. Scanning Electron Microscope (SEM) was utilized to characterize the flank wear patterns. The apparent friction coefficient dealing with the material removal process was deduced from cutting forces measured using Kistler piezoelectric dynamometer. The adhesive frictional signature was examined on both new and worn inserts through series of micro-scratch tests using Atomic Force Microscope (AFM). While abrasion mechanisms dominate the flank wear upon all inserts, the abrasion mode transforms from mild to severe depending upon coating layer characteristics. Regular inspections on the worn face demonstrated that the fail of first-deposited coating layer characterizes a threshold point from which the insert behavior drastically changes. The change in forces and friction tendencies' slopes beyond threshold proves that fiber phase abrasiveness dominates wear mechanisms irrespective to coating type.

Introduction

As well known, success of machining operations depends significantly on the tool properties. Coating type and deposition mode affect the machinability of materials particularly, in case of fiber reinforced polymers (FRP). Single layer coatings were typically composed of titanium carbide (TiC), titanium nitride (TiN) and aluminum oxide (Al₂O₃). The new coating generation obtained from CVD or PVD techniques combines multiple layers such as TiCN, TiN, Al₂O₃ and AlCrO [1], [2]. Titanium-based constituents improve wear resistance to abrasion while oxide constituents are known to offer better chemical stability.

Contrary to metals where plastic deformation dominates the material removal process, more complex and interrelated mechanisms are involved in cutting FRP composites [3]. The changes induced in tool geometry due to wear were known to be of main role on altering its effectiveness on generating high surface quality. Potentially, this results in poor heat localization, significant cutting forces and over degradation in subsurface [4]. Tool grade should be hence rigorously selected in order to resist to fibers' abrasiveness and severe contact loads. Thus, the control of tool wear is a challenging task at different scales. The particles released from the tool and composite phases by effect of interfacial consumption were pointed out as the key of the tribological behavior and wear development.

Among the wide range of available cutting tools, only a few grades meet the stringent requirements of composite machining operations. To circumvent rapid wear and premature failure, the tool material should combine high hardness and high toughness [5]. Since the chip is powdery and able to dissipate only insignificant portion of localized heat, a high thermal conductivity (λ) is outlined as an additional primordial factor in selecting the adequate tool capable of successful FRP cutting [4].

Abrasion, fracture and chipping due to thermal or mechanical loads are the most common mechanisms encountered when cutting CFRP and GFRP [6], [7], [8], [9]. Sakuma et al. [6], [10] have conducted turning tests on both unidirectional CFRP and GFRP using several inserts including carbides, ceramics and cermets. Three wear types were detected on the tool regions; namely (i) rounding wear within the cutting edge, (ii) flank wear within the side-relief angle, and (iii) spalling and/or fracture delamination of the thin film for coated tools. Flank wear arises from the abrasive nature of fibers and the rubbing action between the flank face and the workpiece. The fiber spring-back seem to have sensitive role on intensifying rubbing. According to the authors, the contribution of thermal effects on tool wear might be significant but it still remains incomparable to that of metals' cutting cases. The accumulation of heat within the cutting zone because of reduced thermal conductivities of composite phases (λ_Glass≈1−1.1 W m⁻¹ °C⁻¹, λ_Carbon≈1.7 W m⁻¹ °C⁻¹, λ_Epoxy≈0.2−0.25 W m⁻¹ °C⁻¹) [5], [11] promotes temperature rise (20 °C<T<300 °C) within the tool-tip which might cause nose chipping by thermal effects. When machining FRP, the material release occurs through a series of brittle fractures leading to the formation of discontinuous or powdery chip. Thus, the contribution of the secondary shear zone in heat generation is insignificant or inexistent [12]. This explains the absence of crater wear on tool face when machining polymeric composites. The change in tool geometry because of wear induces undesirable rise of cutting forces and temperature so as to alter the surface integrity and to increase cutting energy consumption [6], [10], [13], [14]. Kim et al. [9] investigated the tool wear according to Taylor criterion when turning carbon/epoxy composite using various speeds and feeds. Edge rounding and flank wear observed have been both attributed to the excessive abrasive action of fibers.

Ferreira et al. [15] reported that diamond tools exhibit the best tool life followed by cubic boron nitride (CBN) and whiskers ceramic tools when turning FRP. Due to the absence of chipping and catastrophic failure, the sintered diamond provides by far the lowest amount of tool wear. Coated as well as uncoated cemented carbides were found suffering from severe flank wear and cutting edge deterioration. The most of flank wear is due to the rubbing action between the flank face and cut fibers within the machined surface. This is shown by the extent of removal of the original grinding marks from the rake face and the roundness of the cutting edge. Rahman et al. [16] carried out fundamental inspections on the wear variation with cutting speed, feed rate and depth of cut using three insert types. Namely, uncoated tungsten carbides, ceramic and cubic boron nitride (CBN) were used to machine both short and long carbon fiber reinforced epoxy composites. The CBN inserts show the best overall turning performance resulting in lowest wear while ceramic inserts reveal vulnerability due to chipping. Close analysis were conducted by Uhlmann et al. [17] on abrasive and adhesive wear of the coated and uncoated carbide versus glass fiber reinforced synthetic material. The diamond-coated insert demonstrated better behavior compared to those uncoated. The diamond layer offers protection not only for in-depth carbide by resisting to the abrasion developed by the action of fibers, but also against thermal wear, resulting basically in erosion at high temperatures. Hence, a coating layers with low thermal conductivity e.g. λ_Ti[C,N]∼26 W m⁻¹ °C⁻¹; λ_TiN∼20 W m⁻¹ °C⁻¹; λ_Al2O3∼34 W m⁻¹ °C⁻¹ [18] are inappropriate for cutting FRP since each of them can form a thermal barrier against energy dissipation which might affect adversely its resistance to wear. Besides hardness and toughness [19], adhesion mode and thermal conductivity of coating layer constituents seem to be hence decisive of the tool lifetime when cutting innovative FRP.

In spite of the advances achieved to overcome inconsistent adhesion of coatings by improving their properties and pretreatment technologies, both single layer and multilayers coated carbides still remain unable to deliver the same benefits as seen in metal machining [15], [17], [20], [21]. The primary limitation of coatings is related to their premature failure by delamination when subjected to abrasion. Adam Khan et al. [22] addressed the wear land on two alumina-based ceramic tools, namely TiCN mixed alumina and SiC whisker reinforced alumina. The measurements were conducted after turning of E-glass fibers reinforced Polyester using variable speed. Observations revealed that both tools underwent progressive wear owing to abrasion. Nevertheless, SiC whisker reinforced alumina grade exhibited slight more efficiency than TiCN mixed alumina grade since abrasive streaks on flank side of the former are smoother.

This attempt discusses the ability of multilayer coatings in cutting GFRP and CFRP composites. Orthogonal cutting tests were, hence, conducted on three inserts with different coating constituents, adherence, grain type and size. SEM inspections were firstly performed on the flank face in order to characterize the wear patterns. The released particles of the both coating layers and composite phases were found of main role in governing the wear progression and friction over the flank face. The measured forces were sensitively correlated with the material removal process in order to explain the changes within the interfaces.