Investigation on the Mechanical Properties of White Layers by Cutting and Burnishing Coupling Effect in BTA Deep Hole Drilling

Abstract

The unique cutting–burnishing coupling effect in BTA deep hole drilling generates a high-hardness and -brittleness white layer (ultrafine martensitic layer), which will degrade component performance and accelerate tool wear. This work investigated the formation mechanism and the mechanical properties of the white layer generated in three distinct regions (the cutting edge radius zone, cutting–burnishing corner zone, and guide pad edge zone) through nanoindentation, SEM and BSE. The microstructure and thickness of the white layer under different feedrates are investigated. The correlations between the white layer, the structure of guide pads, and wear behaviors of the TiN- and TiCN/Al₂O₃-coated guide pads are revealed. Variations in hardness are observed across different zones. The white layer undergoes a soft-to-hard transition due to rapid quenching and the cutting–burnishing effect at the sharp corner. The highest hardness (9.758 GPa) was observed in the guide pad zone, accompanied by grain refinement. The chamfered TiN-coated guide pad exhibits superior wear resistance but suffers fatigue cracking and adhesive wear in the initial guiding zone. The TiCN/Al₂O₃-coated pad with rounded corners experiences brittle spalling in the mid-to-rear guiding zone. These findings enhance the understanding of the white layer formation in deep hole drilling and provide a foundation for tool optimization.

1. Introduction

In modern high-end equipment manufacturing, precision deep hole components such as aero-engine shafts, automotive crankshafts, aircraft landing gear struts, and hollow axles of high-speed trains are vital elements that determine product quality, functionality, and performance, and have become indispensable for national strategic industries [1,2,3]. Benefiting from its unmatched efficiency and stability in ultra-long deep hole drilling [4,5], BTA drilling has become a key process for manufacturing such critical components. However, deep hole drilling takes place under a confined and harsh environment with high-pressure coolant and a dynamically disturbed chip. Its inherent limitations in monitoring, abrupt changes in spatial constraint, risks of process instability, and unique tool structure contribute to the fact that deep hole drilling is not yet extensively understood, particularly regarding its cutting mechanisms and surface integrity. The challenge becomes even more severe when drilling high-strength cast iron materials for crankshaft applications, where the intense thermo-mechanical coupling at the hole wall surface readily induces the formation of a “white layer”, a metastable and non-equilibrium microstructure with ultra-high hardness and pronounced brittleness [6,7]. Acting as a latent “performance killer”, this hardened layer not only significantly reduces fatigue strength [8,9] but also serves as the origin of micro-crack propagation and surface spalling failure [10,11], thereby posing a direct threat to the reliability and safety of critical components, while imposing severe penalties on both production efficiency and economic performance.

The white layer is regarded as the result of the combined effects of austenitic transformation induced by rapid heating and cooling, as well as severe plastic deformation, and its detrimental effects and formation mechanisms have been extensively investigated in traditional machining processes such as hard turning, milling, and grinding [12,13]. The white layers have also been observed on the crests of corrugated steel-rail tracks and wheels, where the contact temperature has been estimated to be well below the austenitization temperature and there is little evidence of localized flow [14]. The principal cause of surface white layers may be associated with machining and high-speed sliding, where rapid heating of the steel surface induces an austenitic transformation, followed by rapid self-quenching by the underlying bulk material [15]. Meanwhile, severe plastic deformation provides an additional driving force for the phase transformation. Machining processing parameters (e.g., cutting speed and feed rate), tool wear state, and machining conditions (dry or wet) have been reported to significantly influence the thickness, hardness, and microstructure of the white layer [16,17,18]. Furthermore, the thickness of the white layer increases progressively with tool wear [19,20,21]. In grinding processes, parameters such as wheel speed have been confirmed to significantly affect the characteristics of the white layer [22,23]. Experimental studies have shown that the mechanical and physical properties of the white layer directly affect the fatigue strength of steel, reduce its resistance to stress corrosion cracking, and enhance its wear resistance under sliding contact [24]. While these findings have provided an important foundation for understanding the behavior of the white layer in conventional machining, research on its formation and characteristics in deep hole drilling remains extremely limited and is still at a contradictory stage. Strodick et al. [25] investigate the effects of cutting speed and feedrate on surface integrity in deep hole drilling and find that high feedrate and cutting speed promote the formation of a white layer on the hole surface. Hayajneh [26] has shown that machining parameters significantly influence hole quality; the surface finish improves with increasing cutting speed and feedrate, but deteriorates once certain thresholds are exceeded. This indicates the existence of an optimal combination of cutting speed and feedrate to achieve the best surface finish. Such apparent contradictions stem from the unique “cutting–burnishing” mechanism in BTA deep hole drilling [27], in which the cutting edge responsibly removes material while the trailing guide pads play a key role in balancing radial forces, smoothing the bore wall, and providing self-guidance [4,28,29]. This coupling mechanism makes the formation of the white layer in deep hole drilling particularly complex. Due to the simultaneous presence of cutting and burnishing zones, the primary thermo-mechanical load generated by the cutting edge induces plastic deformation in the surface layer, significantly increasing its hardness so that subsequent burnishing effects act on already deformed material. Under high-speed rotation, the guide pads continuously burnish the machined surface, generating localized high-pressure and high-temperature fields which induce secondary plastic deformation and phase transformation hardening. This “secondary hardening effect”, which is dominated by the guide pads and plays a key role in shaping the characteristics of the white layer, has been almost completely ignored in traditional machining processes.

Furthermore, the distinctive wear patterns of the BTA tools further complicate the process. Tool wear occurs not only on the main cutting edge but also on the guide pads (e.g., edge blunting and coating delamination), which dominate the dynamic contact interface [30,31]. The characteristics of the white layer generated during drilling also directly affect guide pad wear, creating an interdependent relationship, which in turn leads to another form of contradiction. The wear of the guide pads directly determines the distribution of the frictional contact pressure and heat flux between the guide pads and bore wall, thereby profoundly affecting the intensity of the secondary hardening effect and ultimately controlling the characteristics of the resulting white layer. However, existing studies on tool wear in deep hole drilling have primarily focused on cutting edge wear [32,33,34], and the significant mechanism by which guide pad wear regulates white layer formation through modification of the drilling–burnishing effects has not been thoroughly investigated. Therefore, it is imperative to elucidate how the drilling–burnishing mechanism affects the formation, microstructural evolution, and mechanical properties of the white layer during deep hole drilling. The core objective of this study is to reveal the formation characteristics and mechanisms of the white layer under the drilling–burnishing effects and to establish the causal chain linking guide pad wear, the interface state of the drilling–burnishing, the intensity of the secondary hardening effect, and the properties of the resulting white layer. This link provides essential theoretical support and a basis for process optimization aimed at improving surface integrity and service performance in deep hole drilling, particularly for cast-iron components such as crankshafts

2. Materials and Methods

2.1. BTA Deep Hole Drilling Tests

The experiment employed a T2120 deep hole drilling and boring machine (Dezhou Jutai Machine Tool Manufacturing Co., Ltd.; Dezhou, China) (Figure 1a), with

Table 1. Main technical parameters of T2120 deep hole drilling machine.

Parameter	Unit
Drilling diameter range	Ø20–Ø80 mm
Spindle speed range	61–1000 r/min (12 steps)
Feed speed range	0.005–4.1 mm/rev (stepless)
Oil supply pressure	2.5 MPa

listing the machine’s primary parameters. As demonstrated in Figure 1b, the workpiece was subjected to rotary machining using a Sandvik BTA deep hole tool (Sandvik Group; Sandvik, Sweden) with a diameter of 30 mm. The workpiece material is gray cast iron HT200 with a diameter of 80 mm and a length of 1800 mm. The matrix microstructure is shown in Figure 1c. In order to investigate the formation characteristics of the white layer on different surface regions, factorial experiments were conducted using the parameters listed in

Table 2. Drilling parameters used in this work.

Parameter	Unit	Value
Feed	mm/rev	0.024, 0.0317, 0.044, 0.057
Speed	r/min	630
Depth	mm	100

.

2.2. Sample Preparations

To obtain workpieces containing both cutting and burnishing zones, samples with a hole-bottom morphology were prepared using on quick-stop drilling technique under conditions of 0.024 mm/rev and 630 r/min. As indicated by the dashed lines in Figure 1d, the samples are precisely dissected to access the target analysis regions. Wedge-shaped samples were then prepared (as shown in Figure 1e), preserving three representative cutting zones, the burnishing zone, and the unmachined hole-bottom surface, which also included a cross-sectional profile perpendicular to the hole surface. Further analyses of the wedge blocks included hardness measurements of the white layer on the machined surface, accompanied by SEM-based morphological observations of the wear tracks. White layer hardness data were collected from five distinct regions of each sample, and the subsurface mechanical properties were determined by contacting nanoindentation along the longitudinal cross-sections. The deformation behavior at the micrometer scale was closely examined using BSE imaging within the SEM. To facilitate the hardness testing and BSE measurements, the dissected workpiece was further cut into smaller blocks (as indicated by the arrows in Figure 1b), which were then embedded in an electroconductive resin (Figure 1f) and polished for SEM imaging.

2.3. Measurements and Characterizations

All samples were systematically characterized using a Zeiss Sigma VP Field Emission SEM (Carl Zeiss AG, Oberkochen, Germany) at an accelerating voltage of 15 kV, and observations were performed at multiple magnifications. To ensure imaging quality, the drilled samples were prepared by thermal embedding in an electroconductive resin. Owing to the particular sensitivity of the subsurface microstructure, all samples were polished using an automatic polishing machine under a constant load of 20 N, followed by cleaning with 2% Nitric acid and drying. The final SEM imaging sample is illustrated in Figure 1c. BSE imaging was used to assess the microstructure of polished samples under different drilling conditions. All BSE and Energy Dispersive Spectroscopy (EDS) (JEOL Ltd., Shojima City, Tokyo, Japan) analyses were conducted using a JEOL 7001F SEM at a working distance of 8.5 mm and an accelerating voltage of 25 kV. Nanoindentation was used to determine the hardness distributions in the deformed layers of different zones. The detection site is based on the coverage of the deformed layer and the substrate area (distance from the surface ≥ 50 μm), performed with a Hysitron TI 950 (Bruker Corporation, Minneapolis, MN, USA) TriboIndenter under a load of 5000 μN.

2.4. Drilling and Burnishing Mechanism

Compared with conventional machining methods, BTA deep hole drilling represents a combined process that integrates cutting and burnishing, operating with an internal swarf removing system and a self-guiding action. The drilling process is accomplished through a combined effect of material removal by the cutting edges and elastoplastic deformation exerted by the guide pads against the bore wall. The guide pads (Figure 2b) function as cutting force balancing, guiding, and burnishing the hole surface.

The core mechanism of BTA deep hole drilling lies in the synergistic effect of cutting and burnishing, which enables high-precision machining through the unique design of guide pads. During drilling, the first guide pad (positioned opposite the peripheral- and central-inserts) primarily balances the tangential cutting forces, whereas the second guide pad (collinear with the three inserts) controls the hole diameter in cooperation with the peripheral-insert edge and balances the radial forces. While balancing the cutting forces of the inserts, the guide pads also induced secondary plastic deformation on the machined wall, which induced an effective burnishing effect. This process generates distinct cutting and burnishing zones within the machined areas. The cutting zone is formed by the high-speed rotation of the edge, which removes material and produces a rough initial hole surface, whereas the burnishing zone is formed by the squashing action of the trailing guide pad, which refines the bore wall (Figure 2a). During this process, the material experiences complex elastoplastic deformation, and material ridges are formed at the front edge of the guide pads. Part of these ridges are subsequently removed by the rotating edge of the peripheral insert and further burnished by the guide pads, facilitating material transfer from peaks to valleys. This dynamically balanced cutting–burnishing mechanism, combined with high-pressure coolant cooling during the drilling process, creates a unique thermomechanical environment for white layer formation. During the burnishing process, differences in the guide pad structure and coating (Figure 2b) play a key role in influencing the burnishing mechanics and wear characteristics. As shown in Figure 2c, the chamfered guide pad edge with a prominent geometric transition tends to induce greater material ridges at the leading-contact front, generating a localized stress concentration. This high-stress condition enhances the micro-cutting effects in the initial guiding zone while reducing the overall friction. In contrast, the guide pad edge with a rounded corner produces a gradient distribution of contact pressure, effectively mitigating the stress concentration but significantly increasing the frictional resistance at the rear guiding zone. These structural differences not only alter the burnishing mechanism but also influence the characteristics and distribution of the white layer formed in the burnished zone.

Due to the differences in tool geometry, mechanical characteristics, and thermal behavior between the cutting and burnishing zones, the white layers exhibit different formation characteristics in different zones. Considering the significant impact of the white layer on the quality of the machined surface, this study conducts a comparative analysis of the mechanical characteristics and underlying micro-mechanisms of the white layer in three key contact zones, namely the cutting edge radius zone, cutting–burnishing sharp corner zone, and guide pad edge zone. In addition, the mechanical properties of the deformed layer in the cutting–burnishing planar zone were investigated, and the effect of varying feedrates on the white layer thickness was further investigated.

3. Results and Discussion

3.1. White Layer Formation Characteristics in Different Zones

3.1.1. Cutting Edge Radius Zone

The microscopic morphology of the subsurface in the radius area of the cutting edge after nano-indentation is captured in Figure 3a. This area is precisely the region where the longitudinal section of the workpiece has a distinct circular transition corresponding to the radius of the tool tip, and is marked in red in Figure 3d. In the cutting edge radius zone, the nanoindentation experiments covering 30 measurement points were conducted along the longitudinal depth from the machined surface. The results are shown in Figure 3b, with the red dashed box indicating the indentations in the white layer. It is evident that point A exhibited the highest hardness (7.666 GPa), indicating severe plastic deformation and microstructural refinement in this zone. During deep hole drilling, the combined effects of intense friction and plastic deformation between the cutting edge radius and workpiece material will cause the temperature in the machining area to rise rapidly [35]. Meanwhile, the rapid cooling of sufficient cutting fluid will also lead to surface hardening through quenching. This will promote the dynamic recrystallization of the surface material and the refinement of its microstructure, and cause local phase transformation, eventually forming a highly hard white layer. The hardness at point B is lower, which may be attributed to a relatively weaker stress state and grain deformation. Although point C is closer to the machined surface, its hardness is marginally lower than that of point A, possibly due to the boundary effects.

From the distribution of the elastic modulus, most points remained within the range of 250 ± 25 GPa, whereas Point C at 0.65 μm from the machined surface exhibited a significantly high value of 338.99 GPa. Since the elastic modulus is an intrinsic property of the material, this anomaly is usually regarded as a data artifact, which may arise from two possible factors. First, the indentation at Point C is located near a heterogeneous interface between the material edge and the mounting resin, leading to boundary effects. Such an elastic discontinuity can produce artifacts in the load–displacement curve, thereby distorting the measured data rather than reflecting the true material property [36]. Second, when Point C is located in a region under high compressive stress, material pile-up around the indentation may occur, leading to an apparent increase in the elastic modulus without an actual change [37]. Additionally, as shown in Figure 3c, the load–displacement curve at Point C (green) presents a notable slope change (“elbow”) in the unloading part, which has been reported to be associated with machining-induced amorphization or phase transformation [38], requiring further verification.

3.1.2. Cutting–Burnishing Sharp Corner Zone

In the cutting–burnishing sharp corner zone, as marked by the yellow dot in Figure 4d, the interaction between the cutting edges and guide pads induces material pile-up and forms a ridge structure. The microscopic morphology after nanoindentation tests covering 30 measurement points was carried out throughout the subsurface layer, and the substrate is shown in Figure 4a. The indentation points numbers 0 to 6 marked in the yellow dotted box area are located in the extrusion polishing zone, 7 to 11 are in the cutting zone, and the indentation point No. 7 is positioned near the corner where the two zones intersect. The corresponding test results, shown in Figure 4b, indicate that the nano-hardness in the burnishing zone is generally higher than that in the cutting zone. For example, at a distance of 4.39 μm, point 2 in the burnishing zone (~6 GPa) exhibits a hardness that is approximately 9.09% higher than that of point 10 in the cutting zone (~5.5 GPa). Nano-hardness decreases with increasing distance from the machined surface, and indentations near the surface are noticeably smaller than those in the bulk. The elastic modulus across all points is similar, remaining at approximately 225 ± 25 GPa.

The BSE image (Figure 4c) clearly reveals a distinct white layer in the sharp corner zone. The complex stress state induced by the combined effects of cutting and burnishing, along with rapid quenching at the micro-corner, further promotes dislocation accumulation and local plastic zone hardening [35]. Material pile-ups at the sharp corner lead to localized strain concentration and grain refinement, which may be accompanied by localized martensitic transformation, thereby enhancing the hardness of the white layer.

3.1.3. Guide Pad Edge Zone

When the guide pads apply elastoplastic extrusion to the hole wall, secondary plastic deformation occurs on the machined surface. The high-magnification SEM image (Figure 5a) covering 36-point nanoindentation experiment results was implemented in the guide pad edge zone, which is the area marked by the blue dots in Figure 5d. The relationship between the nanohardness, elastic modulus, which corresponds to the indentation point within the blue dotted line in Figure 5a, and distance from the machined surface was analyzed as shown in Figure 5b. The results show that point A, located closest to the machined surface (0.8118 μm), exhibited the highest nano-hardness of 9.758 GPa. Point A is located within the white layer, where significant grain refinement and increased dislocation density contribute to the elevated hardness. The hardness gradually decreases with increasing distance, stabilizing at approximately 5 GPa at 3.57 μm, and reaching the lowest value of 4.763 GPa at the farthest point B (5.80 μm). The load–displacement curves of points A and B (Figure 5c) directly illustrate this variation. The white layer has a finite thickness, with the hardness gradually decreasing toward the bulk material value as the indentation approaches the underlying substrate. For the elastic modulus, all points remain within 225 ± 25 GPa, except for the measurement at 5.19 μm, which shows a lower value (~150 GPa).

Based on the SEM images, the machined surface in the guide pad edge region (Figure 5a) is smoother than that in the cutting edge radius zone (Figure 3a), reflecting the burnishing effect of the pad edge and resulting in improved surface quality. The hardness of the white layer in this zone is significantly higher than that obtained by cutting alone. This enhancement is attributed to the more severe plastic deformation and thermal effects induced by pad edge burnishing [39,40], which promote surface layer recrystallization, grain refinement, and increased dislocation density. Consequently, a dense and hardened white layer is formed, enhancing the hardness of the surface layer. Therefore, the surface hardening induced by the guide pads is more significant.

3.2. White Layer Characteristics at Different Feedrates

It is no coincidence that similar microstructural zones were obtained at different feedrates. Subsequently, a quantitative analysis was conducted on the extent of the white layer, which has a significant impact on surface integrity, and the results are summarized in Figure 6. Using the contrast of the BSE microscopy, it is noteworthy that the gains were squashed and elongated. The minimum average thickness (approximately 4.326 μm) was found at drilling speed of 630 r/min and feedrate of 0.044 mm/rev (Figure 6b). This layer was also highly uniform. Concurrently, a minimum hardness value (approximately 3.8 GPa) can be achieved. This suggests that more stable and higher-quality drilling–burnishing effects can be generated at an intermediate feedrate. There is no doubt that the softening phenomenon under this cutting condition is significant and is due to the thermo-mechanical mechanism of drilling–burnishing. A smaller hardening layer attributed to the comprehensive action of softening and hardening is not only beneficial for tool life, but also has a lesser influence on subsequent processing. However, a thicker white layer is produced at a low feedrate in Figure 6a, which is due to the lasting plastic deformation rate at a relatively high cutting speed. At a lower feedrate, the contact time between the tool and workpiece is relatively long, and the surface material experiences grain refinement, recrystallization, and phase transformation under thermomechanical effects, which promotes the formation and thickening of the white layer [25]. The EBSD image (Figure 6d) corresponding to Figure 6a employs an inverse pole figure (IPF) color scale, clearly revealing the distribution of grains with different orientations. The sub-grain boundaries and the associated gradients in the IPF color map show well-defined grain distributions from the substrate region and the deformed layers. In contrast, the thickness of the deformation increased again, and the boundaries between the white layer and transitional grain structure layer became blurred, as shown in Figure 6c. This is owing to the excessive burnishing force in the radial direction associated with a large feedrate, which makes it difficult to maintain a good service life for the guide pads. The significantly increased cutting force and strain rate induce rapid localized heating of the material, which further intensifies the thermomechanical effect [41]. In addition, quenching hardening caused by the cutting coolant promotes additional thickening of the white layer.

Further observation of the cross-sectional surface microstructure reveals that the boundary between the white layer and the transitional grain structure (TGS) layer becomes progressively indistinct at a high feedrate, while the grain bands within the TGS layer become finer and exhibit morphological contraction.

3.3. Morphological Characteristics of Guide Pad Wear

Fundamentally, the formation of the white layer results from the combined effects of both cutting force and thermal effects in the cutting–burnishing zone. This process has a significant impact on the wear evolution of the BTA guide pad. As a key drilling component, the frictional behavior (“secondary hardening effect”) of the guide pad is closely related to the characteristics of the white layer.

The wear morphology of the TiN-coated guide pad exhibits a distinct stage evolution during drilling (Figure 7). In the initial wear stage, the pad surface is relatively smooth with only a few sparsely distributed micro-pits and shallow scratches (Figure 7a), resulting from abrasive wear between the hard points of the white layer and the guide pad. As drilling continues, the surface morphology of the guide pad begins to deteriorate, with the abrasive and adhesive wear zones in the initial guiding area and along the bore wall transition from discrete micro-features to concentrated and continuous forms, sharply increasing surface roughness and highlighting wear boundaries, eventually causing coating damage and delamination (Figure 7b). Although the white layer possesses high hardness, its brittleness makes it prone to delamination under sustained high loads and exacerbates adhesive wear [42,43]. Wear initiates in the initial guiding zone rather than at the pad edge due to the chamfered edge design (Figure 2c), which induces localized stress concentrations at the contact front, promoting the formation of dense, hardened white layers during processing [44]. Under continuous rolling-sliding contact, the initial guiding zone bears higher mechanical loads, resulting in more severe wear than in the rear section. Figure 7c presents a magnified view of the dashed-boxed area in Figure 7b, showing fatigue cracks on the surface at the junction, causing spalling of small material fragments and formation of numerous pitted depressions and deep grooves, with considerable workpiece material adhering to the surface defects within the guiding zone.

The elemental distribution shown in Figure 7d provides a clearer indication of the adhesive wear characteristics. Following wear of the guide pad, as a principal component of the TiN coating, the Ti element exhibits pronounced regional attenuation. It remains relatively concentrated in the original coating areas, whereas in severely worn regions, coating delamination leads to a significant reduction in Ti density. Hard phase elements such as W and Ta display localized enrichment at the wear interfaces, reflecting exposure of the substrate due to coating loss. Meanwhile, Fe and C, representative elements of the workpiece, are prominently distributed within the worn zones, indicating intense mechanical interaction between the workpiece and the guide pad surface. During this process, workpiece material is transferred onto the guide pad due to a combination of white layer delamination and adhesive friction. As wear progresses, both the residual distribution and penetration depth of Fe and C increase. In contrast, wear in the rear section of the guide pad (Figure 7e) is primarily associated with adhesive wear caused by micro-pits and scratches.

Figure 8 illustrates the wear morphology and elemental distribution of the TiCN/Al₂O₃-coated guide pad during drilling. In the initial guiding zone, the surface integrity of the Al₂O₃ coating decreases due to its high brittleness, with partial crushing and spalling observed (Figure 8a). This behavior is attributed to the rounded corner design of the guide pad edge, which produces a gradient distribution of contact stress and effectively mitigates stress concentration. The rough surface with layered fractures increases the adhesive wear (Figure 8b,d), with severe local wear zones showing substantial Fe adhesion on the fractured Al₂O₃ surface, compensating for the coating loss, while elements such as Mn and Si are relatively uniformly distributed due to diffusion. However, the edge of the rounded corner design increases the contact area between the guiding zone and the machined surface, significantly raising friction at the rear of the guiding zone. Furthermore, due to the presence of the end land in the guiding zone, the Al₂O₃ coating in the region is continuously disrupted under intense axial feed and rotational-extrusion friction, leading to extensive spalling that exposes the underlying TiCN coating. This results in an inverted-triangle wear morphology with obvious axial extension, indicating that axial stress plays a dominant role in Al₂O₃ coating failure and that the mid-to-rear section of the guide pad bears higher polishing forces (Figure 8c). This behavior contrasts sharply with the wear characteristics observed in TiN-coated guide pads. Ti, C, and N, as the core components of the TiCN coating, remain shielded under intact Al₂O₃ regions but become exposed where the Al₂O₃ coating fails, revealing a degradation pathway of “Al₂O₃ coating barrier failure to TiCN coating exposure” (Figure 8e).

Therefore, the evolution of wear morphology in TiCN/Al₂O₃-coated guide pads results from the combined effects of mechanical abrasion, adhesive-thermal fatigue, and the structural characteristics of the coating. The distinct wear behaviors exhibited by guide pads with different coatings and structural designs provide valuable insights for their optimization, design, and fabrication.

4. Conclusions

• The formation of the white layer at the borehole bottom during deep hole drilling is attributed to the cutting–burnishing effect. Rapid quenching at the sharp corner, combined with the secondary burnishing effect by the guide pad, results in lower hardness on the cut surface (5.5 GPa) compared to the burnished surface (6 GPa). Repeated severe elastoplastic deformation further increases the hardness at the guide pad edge to 9.758 GPa—27.3% higher than that in the cutting edge radius zone. In all regions, the hardness gradually decreases with increasing depth from the machined surface, and grain refinement is identified as the primary strengthening mechanism.
• Within the feedrate range of 0.0317–0.057 mm/rev, the white layer thickness exhibits a U-shaped distribution, which is attributed to the dynamic balance between thermal and mechanical effects during BTA deep hole drilling. At lower feedrate, stronger thermomechanical effects lead to an increased white layer thickness up to 5 μm. At the moderate feedrate (0.044 mm/rev), the layer reaches a minimum thickness of 4.326 μm and exhibits high uniformity.
• The wear behavior of guide pads is strongly influenced by their geometric structure and coating material. The TiN-coated guide pad demonstrated superior wear resistance; however, it experienced fatigue cracking and adhesive wear in the initial guiding zone, ultimately failing due to stress-induced progressive spalling. In contrast, the TiCN/Al₂O₃-coated guide pad features a rounded corner that promotes a more favorable stress gradient, thereby mitigating initial zone damage. Instead, it undergoes adhesive wear and brittle spalling mainly in the mid-to-rear region, following a stepwise degradation process of “Al₂O₃ failure to TiCN exposure”.

Ultimately, a strong correlation exists between guide pad wear and the mechanical attributes of the white layer, with the varying wear characteristics providing essential references for the design and production of deep hole drilling tools.