Optimization of Micro-Sandblasting Parameters for Enhanced Adhesion and Wear Resistance of AlTiSiN-Coated Tools

Abstract

Micro-sandblasting pretreatment was applied to AlTiSiN-coated WC–Co tools to enhance cutting performance in 316 L stainless steel milling. An L₉(3³) Taguchi orthogonal array varied passivation pressure (0.1, 0.2, and 0.3 MPa), gun traverse speed (60, 80, and 100 m/min), and tool rotation speed (20, 30, and 40 r/min). Coating thickness varied only from 0.93 to 1.19 μm, and surface roughness remained within 0.044–0.077 μm, confirming negligible thickness and roughness effects. Under optimized conditions, coating adhesion strength and nano-hardness both exhibited significant improvements. A weighted-scoring method balancing these two responses identified the optimal pretreatment parameters as 0.1 MPa, 80 m/min, and 20 r/min. Milling tests at 85 m/min—using flank wear VBₘₐₓ = 0.1 mm as the failure criterion—demonstrated a cutting distance increase from 4.25 m (untreated) to 12.75 m (pretreated), a 200% improvement. Wear progressed through three stages: rapid initial wear, extended steady wear due to Al₂O₃ protective-film formation and Si-induced oxygen-diffusion suppression, and accelerated wear. Micro-sandblasting further prolonged the steady-wear phase by removing residual cobalt binder, exposing WC grains, and offsetting tensile residual stresses. These findings establish a practical, cost-effective micro-sandblasting pretreatment strategy that significantly enhances coating adhesion, hardness, and tool life, providing actionable guidance for improving the durability and machining performance of coated carbide tools in difficult-to-cut applications.

1. Introduction

Cemented carbide tools are widely utilized in manufacturing due to their superior strength, hardness retention at elevated temperatures, and resistance to plastic deformation, making them highly effective in various cutting processes [1]. However, the increasing use of difficult-to-cut materials in aerospace, automotive, and biomedical industries has significantly challenged conventional carbide tool performance due to accelerated wear and reduced machining efficiency [2].

To address these limitations, applying advanced physical vapor deposition (PVD) coatings, such as titanium aluminum silicon nitride (AlTiSiN), aluminum chromium nitride (AlCrN), and titanium aluminum nitride (TiAlN), has become a primary strategy to improve cutting performance [3]. AlTiSiN coatings, derived from adding silicon (Si) into titanium aluminum nitride (AlTiN), possess exceptional hardness, thermal stability, and oxidation resistance. The presence of optimal Si content (typically 4.7–8 at. %) creates a nanocomposite structure consisting of TiAlN nanocrystals embedded in an amorphous Si₃N₄ matrix, significantly improving hardness (up to 35–57 GPa) and effectively delaying oxidation and thermal degradation at elevated machining temperatures [4]. Additionally, Si improves high-temperature oxidation resistance by forming dense Al₂O₃ and SiO₂ layers, effectively delaying TiO₂ formation [5]. These properties make AlTiSiN coatings ideal for demanding high-speed machining environments [6,7].

Surface pretreatment methods, notably micro-sandblasting, have emerged as crucial techniques for further enhancing coated tool performance. Micro-sandblasting involves high-velocity abrasive particles impacting substrate surfaces, modifying surface morphology, increasing mechanical interlocking, and introducing beneficial compressive residual stresses [8]. Several studies reported significant improvements in coating adhesion, reduced the risk of delamination, and enhanced wear resistance following optimized micro-sandblasting pretreatments. For instance, Bouzakis et al. systematically demonstrated improved cutting performance by optimizing micro-blasting parameters (abrasive material type, blasting pressure, and blasting duration), which resulted in improved coating durability [9]. On the other hand, Puneet C et al. demonstrated that micro-blasting pretreatment, by reducing pre-coating surface roughness and optimizing edge rounding, significantly improved coating adhesion and edge strength, resulting in a threefold increase in the tool life of titanium nitride (TiN)-coated high-speed steel (HSS) tools compared with commercial counterparts [10]. Mechanistically, micro-sandblasting removes surface cobalt binders, exposes tungsten carbide (WC) particles, and induces compressive residual stresses that counteract tensile stresses from sintering, thereby improving interfacial stability. Additionally, the plastic deformation caused by sandblasting generates a high density of twins and dislocations, leading to surface work hardening, narrowing the hardness gap between substrate and coating, and consequently enhancing both the nano-hardness and adhesion strength of the coating [9,10,11].

Despite these developments, most existing research has focused on static parameters such as abrasive material, pressure, and blasting time [8,12,13]. Dynamic micro-sandblasting pretreatment parameters also critically influence substrate morphology and, consequently, coating performance. Passivation pressure controls abrasive particle kinetic energy: pressures below the optimal range may insufficiently activate the surface, limiting adhesion, while pressures above it can induce subsurface damage and excessive residual stress [14]. Gun traverse speed dictates dwell time per unit area, where slower speeds deepen surface texturing and improve mechanical interlocking but risk over-roughening, whereas higher speeds yield more moderate roughness profiles [15]. Tool rotation speed ensures uniform abrasive coverage, with controlled rotation rates preventing localized over-blasting and delivering consistent surface profiles for coating deposition [16].

To systematically evaluate these interdependent factors, we employed a Taguchi L₉(3³) orthogonal experimental design, which enables the efficient estimation of the main effects and interactions with only nine runs [17]. This design has been successfully applied to optimize surface pretreatment parameters in PVD-coated tools, balancing roughness, adhesion, and mechanical integrity while minimizing experimental workload [15]. In this study, the orthogonal array varied passivation pressure (0.1, 0.2, and 0.3 MPa), gun traverse speed (60, 80, and 100 m/min), and tool rotation speed (20, 30, and 40 r/min) to identify the optimal conditions for enhancing AlTiSiN coating adhesion strength and nano-hardness without compromising surface uniformity.

Designated as 022Cr17Ni12Mo2316 L stainless steel is a molybdenum-containing austenitic alloy that offers superior overall properties compared with 310 and 304 stainless steels [18]. However, its machinability is significantly lower—approximately 50% less than that of carbon steels—due to several inherent characteristics. The material’s low thermal conductivity leads to elevated cutting temperatures during machining, resulting in severe tool adhesion and accelerated wear [19]. Additionally, 316 L exhibits a strong tendency for work hardening, which exacerbates tool degradation and further reduces tool life [20]. These factors collectively pose substantial challenges in the machining of 316 L stainless steel.

In this context, this study hypothesizes that optimizing micro-sandblasting parameters will significantly enhance AlTiSiN coating adhesion strength and nano-hardness, thereby substantially improving tool performance when machining 316 L stainless steel. The optimal pretreatment conditions were determined via an orthogonal experimental design and then validated through controlled milling tests and comprehensive wear mechanism analyses.

2. Materials and Methods

2.1. Cutting Tools and Sample Preparation

The cutting tools used were four-flute, straight-shank, flat-end cemented carbide end mills and cemented carbide sample blocks (15 mm × 15 mm × 6 mm) with a nominal composition of 92% WC and 8% Co, supplied by Zhangjiagang Rui Zheng Precision Co., Ltd., Suzhou, China. Tool specifications are listed in

Table 1. Geometric parameters of cutting tools.

Milling Cutter Model	Front Angle	Back Corner	Spiral Angle	Tip Radius	Number of Cutting Edges
Baoke 58° knife	6.5°	11°	40°	3 mm	4

.

2.2. AlTiSiN Coating Preparation

The preparation of AlTiSiN coatings involves three main steps: ultrasonic cleaning, ion etching, and PVD. First, the tool surface is cleaned by high-pressure spraying, ultrasonic washing, and rinsing with pure water to remove oils and contaminants, ensuring a clean and undamaged surface. Then, in the coating chamber, Ar ion etching is performed to clean the substrate and enhance surface activity, followed by Cr target etching to form a hard pseudo-diffusion layer that improves coating adhesion. The etching parameters are set to 100 V bias for Ar etching and 500–800 V bias for Cr etching, with a duration of 300 s. Finally, the AlTiSiN coating is uniformly deposited on the substrate surface by using PVD, resulting in a coating with excellent wear resistance and corrosion resistance, widely applied in industrial fields. Compared with the aluminum chromium (AlCr) target in AlCrN coatings, the Si in the TiSi target is more difficult to ionize, requiring the coating furnace to be heated to 480 °C to ensure effective Si doping [21].

2.3. Materials and Micro-Sandblasting Pretreatment

In this study, flexible abrasives composed of Al₂O₃ particles coated with a carbon-based material were used for micro-sandblasting pretreatment (Figure 1). Specifically, Figure 1A,B show Al₂O₃ abrasive particles coated with carbonaceous material. Unlike conventional abrasives, which are either spherical (e.g., ZrO₂) or sharp-edged (e.g., traditional Al₂O₃) [22], the abrasives used here exhibit irregular shapes without sharp edges, featuring surface protrusions and cracks, with particle sizes predominantly ranging between 1 and 2 mm. While spherical abrasives (such as ZrO₂) typically induce pit-like features on substrate surfaces, sharp-edged Al₂O₃ abrasives usually fracture upon impact, generating sharper edges and intensifying their cutting action. The uniquely coated and flexible Al₂O₃ abrasives depicted in Figure 1A,B, however, balance efficient surface treatment with minimal damage to cutting tool edges, preventing common issues such as edge chipping.

Micro-sandblasting modifies the substrate’s surface morphology and roughness, promoting uniformity and enhancing mechanical interlocking between the coating and substrate, thereby improving coating adhesion. In this study, micro-sandblasting pretreatment was performed using a PM-AN1200 fully automatic sandblasting machine (Shenzhen Xinyisheng Technology Co., Ltd., Shenzhen, China) (Figure 2). Three independent micro-sandblasting and coating runs were performed (n = 3) to ensure process reproducibility. Within each run, surface roughness, thickness, nano-hardness, and scratch critical load (adhesion) were each measured at five distinct, randomly chosen locations (n = 5), and the averages were used for analysis.

2.4. Orthogonal Experimental Design Scheme

We used a Taguchi L₉(3³) orthogonal array [17] to evaluate the effects of three micro-sandblasting parameters—pressure, gun traverse speed, and tool rotation speed—each at three levels, to optimize the micro-sandblasting pretreatment parameters, with the blasting time fixed at 20 s. This design allows for the efficient estimation of the main effects with only nine experiments. The factor levels were chosen based on preliminary trials and the literature [14]: pressure (MPa): 0.1, 0.2, and 0.3; movement speed (m/min): 60, 80, and 100; tool rotation speed (r/min): 20, 30, and 40. These settings span a representative operating window for micro-sandblasting pretreatment.

2.5. Characterization of Coating Microstructure

Surface Morphology Analysis of Coating: Following micro-sandblasting pretreatment, the tools and samples were cleaned using ultrasonic cleaning. The surface morphology and cutting-edge passivation were analyzed using scanning electron microscopy (SEM; JSE-6510A).

Coating Thickness Measurement: The coating thickness of AlTiSiN-coated cemented carbide specimens, deposited after micro-sandblasting pretreatment, was measured using a ball mill tester. A stainless steel ball with a diameter of 6 mm was used as the counter ball, with diamond spray as the grinding medium. The coated specimens were fixed, and the milling parameters were set to a rotational speed of 320 r/min and a grinding duration of 20 s.

Coating Surface Roughness: The surface roughness of the samples was measured using the SJ5800-200 integrated profilometer. A stylus moved linearly across the sample surface under a controlled force, driving a high-precision displacement sensor vertically to capture surface height variations. This enabled the measurement of roughness parameters Ra and Rz. Each sample was measured five times, and the average value was recorded.

2.6. Coating Mechanical Property Testing

The mechanical properties of coatings are closely related to the cutting life of hard-alloy coated tools, which is primarily reflected in adhesion strength and nano-hardness.

Nano-Hardness: In this study, a DHV-1000Z digital microhardness tester was used to measure the Rockwell hardness of the coatings prepared with different micro-sandblasting pretreatment parameters. The effects of passivation pressure, gun traverse speed, and tool rotation speed on coating hardness were investigated. Measurements were conducted on the coating surface with a normal load of 0.5 N and a dwell time of 15 s. Nanoindentation tests were carried out on the coating surfaces by using a Nano Indenter G200. A maximum normal load of 2 N was applied to determine the nano-hardness of the multilayer films. Each sample was tested five times randomly under the same load, and the average value was taken.

Coating Adhesion Strength: This study utilized a WS-2005 automatic scratch tester to perform scratch tests on coated hard-alloy samples, observing the dynamic behavior of multilayer films under linearly increasing loads. Precise values of coating adhesion strength were obtained, and the effects of micro-sandblasting pretreatment parameters on coating adhesion were investigated.

Analysis of Variance (ANOVA): A univariate ANOVA was performed with SPSS Statistics (v. 26) to assess the influence of micro-sandblasting parameters on coating binding strength, nano-hardness, and their combined performance. For each of the nine L₉(3³) experimental runs, the mean values of binding strength (scratch test) and nano-hardness were entered as dependent variables. The three factors—passivation pressure, gun traverse speed, and tool rotation speed—were specified as fixed factors with three levels each. SPSS computed the Sum of Squares (SS), degrees of freedom (DF), mean squares (MS = SS/DF), and F-values for each factor and the error term.

Range Analysis: We used SPSS Statistics (v. 26) to obtain the mean values of the response at each level of the three factors (pressure, movement speed, and tool rotation speed) via the Explore procedure. The range R for each factor was then calculated as the difference between its maximum and minimum level means, with larger R indicating a stronger influence. In Taguchi’s methodology, larger R values denote greater influence on performance. Range analysis thus identifies the primary and secondary effects of each factor on mechanical properties, while ANOVA offers supplementary quantitative insights into their relative contributions. Combined, these methods robustly pinpoint the most influential parameters even when the formal ANOVA lacks statistical power.

Comprehensive Performance Analysis: To integrate adhesion strength and nano-hardness into a single performance index, a comprehensive weighted-scoring method was applied, which is widely used for the multi-objective optimization of coating processes [23].

The comprehensive weighted-scoring formula is as follows:

$$Y_i=m_{i1}{n}_{i1}+m_{i2}{n}_{i2}+\dots +m_{ij}{n}_{ij}$$

(1)

where m_ij is the weighting coefficient, representing the proportion of each index in the weighted score; n_ij is the experimental index value; and subscripts i and j represent the JTH index value of the experiment in group i. Kj is the difference between the maximum value and the minimum value of each group of experimental indicators; then, the values of K₁ and K₂, are

$$K_1=74-48=26$$

(2)

$$K_2=3525-2602=923$$

(3)

In this work, two responses were considered: adhesion strength and nano-hardness. Adhesion strength and nano-hardness were both assigned a weight of 50 out of 100 (equal importance), following the recommendation that both adhesion and hardness contribute comparably to tool life when the coating thickness and roughness are held constant [24].

$$m_{i1}={\displaystyle \frac{50}{K_1}}=1.923 m_{i2}={\displaystyle \frac{50}{K_2}}=0.054$$

(4)

In summary, the comprehensive performance evaluation formula, denoted by Y, was established.

$$Y_1=1.923n_{i1}+0.054n_{i2}$$

(5)

2.7. Cutting Test on 316 L Stainless Steel

Under the optimal micro-sandblasting pretreatment parameters, coated tools with different forward-to-reverse rotation time ratios (1:1, 3:4, and 4:3) were used to perform the unidirectional down-milling of 316 L stainless steel plates on a Taiwan CHANG CHUN E-600 CNC (Taiwan, China) vertical milling machine with fixed cutting parameters (

Table 2. Cutting parameter of Taiwan Normall E-600 CNC vertical milling machine.

Spindle Speed (r/min)	vc (m/min)	vf (mm/min)	fz (mm)	ap (mm)	ae (mm)	Overhang Length (mm)
4500	85	1500	0.09	0.3	2.5	30

). All milling tests reported in this work were carried out under dry machining conditions (no coolant or lubricant). Cutting was stopped when the spindle load monitoring (SLM) value reached 3, and the tool life was recorded. The tool morphology and lifespan were recorded accordingly. During the cutting process, the tool’s flank and rake face wear were measured every 1 min using a 2D video measuring system. Tool failure was defined when the maximum flank wear width (VBₘₐₓ) reached 0.1 mm [25]. This criterion was applied uniformly to both untreated and micro-sandblasting-pretreated tools, ensuring a direct comparison of their wear resistance and cutting life under the same testing protocol. Milling tests on 316 L stainless steel were also carried out in triplicate for both untreated and optimized–pretreated tools, and the mean cutting distance at VBₘₐₓ = 0.1 mm is reported. Upon completion of each cutting test, the tools were ultrasonically cleaned, and the wear morphology on the tool surfaces was examined using SEM. Additionally, an energy-dispersive X-ray spectrometer (EDS) was employed to analyze the elemental composition in the wear regions, enabling the identification of wear mechanisms and modes.

The 316 L stainless steel workpiece used in the tests measured 300 mm × 200 mm × 50 mm and had a Rockwell hardness of 28 ± 1 HRC. Its chemical composition and mechanical properties are listed in

Table 3. Chemical composition of 316 L stainless steel (%).

Elements	Cr	Ni	Mo	Mn	Si	C	P	Fe
Composition Ingredients	16 to 18	10–14	2–3	2 or less	1 or less	0.03 or less	0.045 or less	Other

and

Table 4. Basic characteristics of 316 L stainless steel.

Density (g/cm3)	HRC	Melting Point (°C)	Heat Conductivity (20–100 °C) (W/(m·K))	Deformation Temperature (°C)	Tensile Strength (MPa)	Elongation (%)	Reduction in Section (%)
7.98	28	1400	15	800	530	40	60

, respectively.

3. Results and Discussion

3.1. Effect of Micro-Sandblasting Pretreatment Process Parameters on Microstructure of AlTiSiN Coatings

3.1.1. Micro-Sandblasting Pretreatment Has Minimal Effect on Surface Roughness of AlTiSiN Coating

To improve the cutting performance of AlTiSiN-coated cemented carbide tools, this study employed an orthogonal experimental design to investigate the effects of sandblasting pretreatment parameters—namely, passivation pressure, gun traverse speed, and tool rotation speed—on the surface roughness, adhesion strength, and nano-hardness of AlTiSiN coating. Based on the optimized sandblasting conditions, AlTiSiN-coated carbide tools with varying forward-to-reverse rotation time ratios were prepared and tested by machining 316 L stainless steel, thereby identifying the optimal sandblasting pretreatment process (Figure 3A). The AlTiSiN coating was developed by extending the AlTiN coating process and simultaneously using two targets, AlTi and TiSi, to achieve silicon doping (Figure 3B). In this study, the AlTiSiN coating was prepared using target materials with a Ti-to-Al atomic ratio of 33:67 and a Ti-to-Si atomic ratio of 85:15. The purity of all target materials was 99.8%. This coating was applied to four-flute, straight-shank, flat-end cemented carbide end mills, as well as cemented carbide samples of the same material (Figure 3C). After the cemented carbide samples were coated, the AlTiSiN-coated samples underwent micro-sandblasting pretreatment according to the parameter combinations listed in

Table 5. Surface roughness results of coatings after micro-sandblasting pretreatment under different parameters in orthogonal experimental design.

Group	Factor 1	Factor 2	Factor 3	Surface Roughness (μm)
	Pressure (MPa)	Movement Speed (m/min)	Tool Rotation Speed (r/min)
1	0.1	60	20	0.0652
2	0.1	70	30	0.0638
3	0.1	80	40	0.0581
4	0.2	70	40	0.0572
5	0.2	80	20	0.0680
6	0.2	60	30	0.0499
7	0.3	80	30	0.0720
8	0.3	60	40	0.0692
9	0.3	70	20	0.0448
R Value	0.00400	0.01077	0.00257	-

. Subsequently, the surface roughness of the coatings was measured using a profilometer. Each group of samples was measured five times, and the average value was taken (Table 5).

As shown in Table 5, the surface roughness of the AlTiSiN coatings under different process parameters ranges from 0.0448 to 0.0772 μm. Among them, coatings processed under the parameters of Group 6 and Group 9 achieved relatively lower surface roughness values of 0.0448 μm and 0.0499 μm, respectively. Overall, the micro-sandblasting pretreatment has a limited effect on the coating’s surface roughness. This is mainly because, during deposition, the high-temperature arc causes the rapid evaporation of the material near the molten pool, ejecting molten droplets of varying sizes that form a relatively stable surface morphology [26]. The surface roughness shows a nonlinear variation with changes in passivation pressure, gun moving speed, and tool rotation speed. Range analysis indicates that gun moving speed is the primary factor affecting surface roughness, followed by passivation pressure, while tool rotation speed has the lowest impact (Figure 3D). In summary, the micro-sandblasting pretreatment parameters have a limited impact on the surface roughness of AlTiSiN coatings, with gun moving speed being the dominant factor.

3.1.2. Micro-Sandblasting Treatment Improves Tool Surface Quality and Coating Adhesion

We found that macroscopically, the flank face and side edge of AlTiSiN-coated cemented carbide tools without micro-sandblasting pretreatment appeared relatively smooth, with no obvious chipping. However, scanning electron microscopy revealed that these untreated tool surfaces exhibited grinding scratches, pits, and slight micro-chipping along the cutting edge. Such defects can reduce the coating’s adhesion strength during subsequent deposition and make the cutting edge prone to chipping, ultimately shortening tool life (Figure 4A). In contrast, after micro-sandblasting pretreatment with different parameters, the tool surfaces became smoother, with most micro-defects at the cutting edges effectively removed (Figure 4B). This significantly improved the edge surface quality, which benefits the adhesion performance of the deposited coatings and helps mitigate edge chipping and wear during cutting operations. Additionally, a small number of residual particles were observed on the tool substrates. However, these can be effectively eliminated by ultrasonic cleaning and the metal target etching process during coating deposition, thereby minimizing their impact on coating quality.

3.1.3. Micro-Sandblasting Has No Effect on Surface Coating Thickness of AlTiSiN

Further coating thickness measurements of the AlTiSiN-coated cemented carbide samples before and after micro-sandblasting pretreatment were performed using a ball mill tester. The results showed that the coating thickness on the samples without micro-sandblasting pretreatment was 1.01 μm. The Rockwell indentation profile was smooth and intact, with a coating adhesion rating of HF1, featuring rounded pits without chipping or visible cracks (Figure 5A,B). As shown in Figure 5C, the coating thickness of the AlTiSiN-coated tools after micro-sandblasting pretreatment ranges from 0.93 μm to 1.19 μm, with all values falling within the experimental error margin of ±0.2 μm. This result indicates that the micro-sandblasting pretreatment, under the parameters studied, does not cause any significant change or deviation in coating thickness. It is well documented that coating thickness can strongly influence the performance of cutting tools, including their wear resistance, adhesion, and thermal stability [27,28]. Excessively thick or uneven coatings may lead to higher residual stress or reduced adhesion, both of which can adversely affect tool life and reliability [29]. By confirming that the coating thickness remains nearly constant across all samples and treatment conditions, this study effectively isolates the impact of micro-sandblasting on other critical coating properties. Thus, any differences observed in tool wear, cutting life, or performance in subsequent analyses can be attributed primarily to changes in coating microstructure, hardness, and coating–substrate adhesion, rather than variations in coating thickness.

3.2. Influence of Micro-Sandblasting Pretreatment Parameters on Mechanical Properties of AlTiSiN Coatings

The mechanical properties of coatings are closely related to the cutting performance and service life of coated cemented carbide tools, which is primarily reflected in two aspects: adhesion strength and nano-hardness [30]. Among these, coating–substrate adhesion strength is one of the key criteria for evaluating the cutting performance of coated tools. In this study, a scratch test was employed to observe the dynamic behavior of multilayer films under linearly increasing loads and obtain the specific adhesion strength values of the coatings. Meanwhile, the hardness of a coating is closely related to its microstructure and phase composition, and it serves as an important parameter for assessing the coating’s resistance to plastic deformation. Initially, a DHV-1000Z digital microhardness tester was used to measure the Rockwell hardness of coatings prepared under different micro-sandblasting pretreatment parameters. However, due to the minor variations and relatively large experimental errors in Rockwell hardness, it was difficult to clearly reflect the influence trends of passivation pressure, gun moving speed, and tool rotation speed on coating hardness. Therefore, further nanoindentation tests were conducted using a Nano Indenter G200 system, with a maximum normal load of 2 N applied to measure the nano-hardness of the multilayer coating surfaces.

The results revealed that the nano-hardness and adhesion strength of AlTiSiN coatings deposited with different micro-sandblasting parameters showed certain differences (

Table 6. Summary of orthogonal experiment evaluation for coating mechanical properties.

Groups	Factor 1	Factor 2	Factor 3	Coating Mechanical Properties
	Pressure (MPa)	Movement Speed (m/min)	Tool Rotation Speed (r/min)	Binding Force (N)	Nano-Hardness (HV)	Comprehensive Weighted Scoring
1	0.1	60	20	57	3525	299.961
2	0.1	70	30	54	3314	282.798
3	0.1	80	40	63	3268	297.621
4	0.2	70	40	48	3079	258.570
5	0.2	80	20	65	3162	295.743
6	0.2	60	30	74	2602	282.810
7	0.3	80	30	61	3130	286.323
8	0.3	60	40	61	3297	295.341
9	0.3	70	20	64	3038	287.124

). Range and variance analyses further indicated that gun moving speed had the most significant effect on coating adhesion strength, while passivation pressure was the dominant factor influencing nano-hardness (

Table 7. ANOVA for coating properties in Table 6. Sum of Squares (SS), degrees of freedom (DF), mean square (MS), and F-values for binding strength, nano-hardness, and overall performance were computed directly by SPSS.

Coating Properties	Factors	SS	DF	MS	F-Value	p-Value
Binding Power, N	Passivation pressure (A)	34.889	2	17.444	0.169	0.856
	Gun movement speed (B)	134.889	2	67.444	0.625	0.605
	Tool rotation speed (C)	54.889	2	27.444	0.265	0.790
	Error	206.889	2	103.444
Nano-hardness, HV	Passivation pressure (A)	266,304.889	2	133,152.444	1.674	0.374
	Gun move speed (B)	3909.556	2	1954.778	0.025	0.976
	Tool rotation speed (C)	91,689.556	2	45,844.778	0.576	0.634
	error	159,113.556	2	79,556.778
Overall rating	Passivation pressure (A)	334.246	2	167.123	2.152	0.317
	Gun movement speed (B)	565.061	2	282.531	3.638	0.216
	Tool rotation speed (C)	214.914	2	107.457	1.384	0.420
	Error	155.332	2	77.666

and

Table 8. Range (R) values from the orthogonal experiment in Table 6, calculated using SPSS.

Mechanical Properties	Pressure	Movement Speed	Tool Rotation Speed	Optimal Process Parameter Combination
Binding force	4.33	8.67	5.67	Passivation pressure: 0.1 MPa Gun moving speed: 80 m/min Knife rotation speed: 20 r/min
Nano-hardness	421	45	226
Comprehensive Score	14.4	17.1	10.4

). The visual analysis of adhesion strength (Figure 6A) showed that with the increase in passivation pressure and tool rotation speed, the adhesion strength initially increased and then decreased; as the gun moving speed increased, the adhesion strength first decreased and then increased. Similarly, the visual analysis of nano-hardness (Figure 6B) demonstrated that as passivation pressure and tool rotation speed increased, the nano-hardness decreased first and then increased, while the nano-hardness gradually increased with the increase in gun moving speed.

To identify the optimum micro-sandblasting pretreatment, we applied a multi-criterion weighted-scoring method to the orthogonal array results in Table 6. Because (i) all tools received the same AlTiSiN coating, (ii) the post-coating thickness remained within a narrow band, and (iii) the WC–Co substrates were identical, the two responses that most strongly govern cutting performance are binding force (adhesion strength) and nano-hardness. Coating thickness and roughness effects were, therefore, considered negligible. Numerous studies have reported that an increase in hardness often correlates with reduced adhesion due to higher residual stresses; therefore, the best overall tool performance is achieved by balancing these two competing attributes rather than maximizing either one independently [24]. To capture this trade-off, we employed the weighted-sum method, which is widely used for the multi-objective optimization of coating processes [23]. For each experimental run i in the L₉(3³) orthogonal array, the comprehensive performance index ${\mathrm{Y}}_{\mathrm{i}}$ was calculated as a weighted sum of the normalized responses (Table 6). The weighted-scoring algorithm and its justification are described in Materials and Methods, Section 2.6. The ANOVA indicates that none of the factors—passivation pressure, gun traverse speed, or tool rotation speed—exceeded the conventional significance threshold (F  <  4, p  >  0.05) (Table 7), a consequence of the small error DF (2) in our nine-run design. To overcome this limitation, we applied Taguchi range analysis, which ranked gun traverse speed as the most influential factor, followed by pressure and rotation speed (Table 8). A complementary visual analysis (Figure 6C) confirmed that coating performance first decreased and then improved with the increase in passivation pressure and gun traverse speed, while it steadily declined with higher rotation speeds.

Based on these findings, the optimal micro-sandblasting pretreatment parameters were determined as follows: a passivation pressure of 0.1 MPa, a gun moving speed of 80 m/min, and a tool rotation speed of 20 r/min.

3.3. Effect of Forward-to-Reverse Rotation Time Ratio on Cutting Life of Coated Tools

SEM is an essential tool for analyzing the surface morphology of PVD hard coatings [31,32]. Therefore, in this study, SEM was employed to characterize the surface morphology of the AlTiSiN-coated samples. The comparison of surface morphologies revealed that the untreated AlTiSiN-coated samples exhibited scratches, numerous irregularly sized droplets, and pores (Figure 7A). These defects served as diffusion pathways for oxygen and elements during high-temperature oxidation, accelerating tool wear and reducing cutting life. Additionally, large droplets could induce coating cracking or act as abrasive particles during cutting, further deteriorating tool performance and affecting the machined surface quality. In contrast, micro-sandblasting pretreatment under optimized parameters (0.1 MPa, traverse speed of 80 m/min, and rotation speed of 20 r/min) effectively reduced surface defects such as droplets and pores, resulting in a smoother and more uniform coating surface (Figure 7B). This improvement in surface condition significantly enhanced the overall mechanical and functional performance of the coating by providing a more favorable deposition environment. While the SEM analysis in this study clearly demonstrates the reduction in surface defects and improved uniformity resulting from micro-sandblasting, it inherently provides only two-dimensional, localized information. The lack of three-dimensional (3D) profilometry limits our ability to quantitatively assess critical surface parameters such as areal roughness and peak-to-valley height, thus constraining the depth and comparability of our surface characterization. Furthermore, previous studies have established that micro-sandblasting can effectively alter the microstructure of hard coatings—by refining grain size, introducing residual compressive stress, and modifying surface morphology—which in turn significantly affects mechanical properties such as hardness and adhesion strength [33,34,35]. In the present study, the observed improvements in mechanical performance (e.g., enhanced adhesion strength and nano-hardness) following micro-sandblasting pretreatment are likely attributable to these microstructural modifications. However, direct microstructural analyses—such as X-ray diffraction for phase identification or SEM/TEM for grain size and interface observation—were not conducted. As a result, the structural origins underlying the observed mechanical property changes could not be explicitly demonstrated.

This constitutes a limitation of the current work. To fully elucidate the relationship between micro-sandblasting-induced structural changes and the resulting mechanical property enhancements, further microstructural characterizations will be necessary in future studies.

After grinding, cutting tools often exhibit varying degrees of microscopic defects along the cutting edge, which can easily propagate during machining, accelerating tool wear and failure. Edge passivation is an essential auxiliary technique in metal cutting, effectively minimizing or eliminating these micro-defects, resulting in a smoother, more uniform edge, enhanced impact resistance, and extended tool life. Micro-sandblasting pretreatment not only removes surface contaminants and micro-defects from cutting tools, thereby improving coating performance, but also serves to passivate the cutting edge [36,37]. Appropriate passivation parameters can eliminate edge defects, optimize edge morphology and radius, and reduce residual stress and surface cracks that may cause edge fatigue [38].

In this study, single-factor cutting tests were conducted under optimized micro-sandblasting parameters to investigate the effect of forward-to-reverse rotation time ratios on coated tool life. Using a Taiwan CHANG CHUN E-600 CNC vertical milling machine, the unidirectional down-milling of 316 L stainless steel was performed with constant cutting parameters. Tool life was determined as the time taken for the SLM to reach a value of 3. The results showed that a 4:3 forward-to-reverse ratio yielded the longest tool life of 17 min 30 s, compared with 9 min 52 s for untreated tools.

Wear analysis revealed that tool wear was predominantly on the rake face, with minor flank wear mainly characterized by coating delamination. During forward rotation, more abrasive particles impacted the rake face, while reverse rotation affected the flank face. At 20 s of micro-sandblasting duration, increased forward-pass time improved tool life by refining the deposition environment, inducing work hardening in the substrate, and enhancing coating hardness and wear resistance. In the milling of 316 L stainless steel, the control group tool exhibited severe chipping and substrate exposure on the flank face (Figure 8A). In contrast, with micro-sandblasting pretreatment at a forward-to-reverse rotation time ratio of 1:1, only slight chipping occurred on the flank face without substrate exposure (Figure 8B). When the ratios were 3:4 and 4:3, the tools showed minimal wear and excellent performance (Figure 8C,D). Notably, local chipping was observed on the rake face of Tool C. Overall, the tool pretreated with a 4:3 forward-to-reverse rotation time ratio demonstrated the best wear resistance, minimal damage, and superior cutting performance.

3.4. Optimized Micro-Sandblasting Pretreatment Enhances Wear Resistance of AlTiSiN-Coated Tools During 316 L Stainless Steel Milling

3.4.1. Flank Wear Behavior of AlTiSiN-Coated Tools During 316 L Stainless Steel Milling

During the milling of 316 L stainless steel, the wear process of AlTiSiN-coated carbide tools can be divided into three distinct stages: initial wear, steady wear, and rapid wear [39]. In the initial stage, the wear rate is relatively high, but the duration is short. As the process enters the steady-wear stage, the Al in the coating reacts with the oxygen in the air to form a protective Al₂O₃ film, which mitigates oxidative wear and protects the tool surface under elevated temperatures [40]. Meanwhile, the Si element within the coating effectively suppresses oxygen diffusion, reduces the coefficient of friction, and slows the wear rate, thereby extending the duration of the steady-wear phase [41]. However, as the cutting temperature continues to rise, severe friction induces cold welding on the rake face, leading to the formation of built-up edges (BUEs). The repeated formation and detachment of these BUEs result in coating delamination and the formation of pits on the exposed substrate surface. When the frictional force exceeds the bonding strength between the coating and substrate, chip erosion intensifies the wear process [39,42]. In addition, the high affinity between the Fe, Cr, Mn, and Ni in 316 L stainless steel and the Co in the tool substrate promotes diffusion wear and adhesive wear, accelerating tool failure [39].

Thus, tools without micro-sandblasting pretreatment exhibited severe wear characteristics on the flank face, including substrate delamination, edge chipping, scratches, adhesion, and BUE formation (Figure 9A). The energy-dispersive spectroscopy (EDS) analysis of Area 6 revealed the presence of Cr, Fe, Ni, Mo, and C from the workpiece, indicating serious cold welding at this location (Figure 9B). In Area 7, W, C, and Co originated from the tool substrate, while Fe, Cr, and Ni derived from the workpiece, indicating the coexistence of oxidation, adhesion, and abrasive wear mechanisms (Figure 9C). The abrasive wear originated from hard inclusions in the workpiece surface, chip fragments, and detached BUE debris.

In contrast, the flank wear band on the micro-sandblasted tool appeared smoother and more uniform, with predominantly adhesive wear observed on the flank face (Figure 9D). The EDS analysis of Area 8 confirmed coating delamination and the occurrence of oxidation, adhesion, and diffusion wear (Figure 9E). Adhesive wear, or cold welding wear, occurs when cold-weld points are sheared under high temperature and pressure during relative motion between the tool and the workpiece, causing material particles from the tool to detach.

To further assess the influence of micro-sandblasting pretreatment on the cutting performance of AlTiSiN-coated tools, additional milling tests were performed on 316 L stainless steel at a cutting speed of 85 m/min. The tool flank wear was measured using a 2D video measurement system, with a maximum flank wear width (VBmax) of 0.1 mm set as the tool failure criterion. A graph of flank wear versus cutting time for the wet-milling of 316 L stainless steel with AlTiSiN-coated carbide tools was plotted (Figure 9F). At the failure criterion (VBmax = 0.1 mm), the untreated coated tool achieved a cutting length of only 4.25 m, while the optimally micro-sandblasted tool reached an impressive cutting length of 12.75 m. Moreover, the steady-wear stage for the micro-sandblasted tool was significantly prolonged. This improvement is attributed to the removal of the cobalt binder from the substrate surface by micro-sandblasting, which increased the exposure of tungsten carbide grains. Additionally, the work-hardening effect induced by micro-sandblasting partially offset residual tensile stresses, thereby enhancing the mechanical properties of the substrate surface and the coating adhesion strength. In the rapid wear stage, the untreated tool exhibited a sharp increase in wear after 4 min, including edge chipping and substrate spalling. In contrast, no obvious rapid wear was observed for the micro-sandblasted tool before reaching a flank wear of 0.1 mm.

In summary, these results demonstrate that the optimized micro-sandblasting pretreatment parameters significantly improved the cutting performance of AlTiSiN-coated tools, extended the steady-wear stage, and prolonged tool life.

3.4.2. Wear Morphology of Rake Face of AlTiSiN-Coated Tools During Milling of 316 L Stainless Steel

The SEM observation of the rake face of AlTiSiN-coated tools after milling 316 L stainless steel revealed that tools without optimized micro-sandblasting pretreatment exhibited significant edge chipping, severe wear, coating delamination, fine surface scratches, and adhesion (Figure 10A). A further EDS analysis of selected Area 1 on the rake face showed high oxygen content, indicating pronounced oxidative wear, with primary oxides likely including Al₂O₃, WO₃, and Fe₂O₃ (Figure 10B). This oxidative wear typically occurs at cutting temperatures of 700–800 °C, where elements within the carbide substrate react with the oxygen in the air to form a low-hardness oxide film. Such films tend to accumulate at the tool–workpiece interface and are gradually peeled off due to friction from the oxide scale, work-hardened layer, and hard inclusions during cutting, exacerbating wear. Additionally, the detection of Cr, Fe, and Ni in Area 1 indicates adhesive wear caused by material bonding between the workpiece and tool rake face. Edge chipping results in a blunt cutting edge and reduced cutting efficiency, making the tool surface more prone to adhesive and diffusion wear under high temperature and pressure, accelerating tool degradation, shortening tool life, and compromising workpiece surface quality. The EDS analysis of Area 2 revealed primarily Fe, C, and O, and minor Cr, Ni, Mo, and W, indicating coating breach and substrate exposure. The oxygen content here was lower than in Area 1, suggesting that oxidative wear is concentrated near the cutting edge and diminishes with distance from the edge (Figure 10C). Area 3, located further from the cutting edge, contained Ti, Al, O, and N, and minor Fe, W, Cr, Co, and Ni (Figure 10D). The oxygen content further decreased, while nitrogen was abundant, indicating a relatively intact coating. Under high temperature, aluminum forms a dense Al₂O₃ film that inhibits oxygen diffusion into the coating and metal ion outward diffusion, enhancing oxidation resistance. However, this film is progressively damaged with continued cutting. Fe mainly originates from the workpiece and facilitates high-temperature diffusion wear. This involves the mutual diffusion of W and C atoms with Fe, causing elemental depletion or enrichment on the tool surface, reducing atomic bonding strength and wear resistance, and ultimately shortening tool life.

In contrast, the AlTiSiN-coated tools subjected to optimized micro-sandblasting pretreatment exhibited lighter wear and intact cutting edges after milling 316 L stainless steel (Figure 10E). Although some coating wear and minor delamination were observed, only slight scratches and mild adhesion appeared on the surface. This improvement is attributed to the micro-sandblasting process, which enhances the coating deposition environment and induces plastic deformation on the carbide substrate surface, generating abundant twins and dislocations. This results in work hardening, increased surface hardness, and improved microscopic hardness of the coating, thereby strengthening the multilayer structure of the AlTiSiN coating, enhancing wear resistance and thermal stability, and extending tool life. The EDS analysis of Area 4 on the rake face of the micro-sandblasted tool showed predominant elements of Fe, W, Ti, C, and O, with minor Al, Cr, Co, and Ni, indicating oxidative wear with the formation of TiO₂ and Fe₂O₃. TiO₂ increases surface hardness, reduces friction, and protects the tool from oxidation and corrosion, improving tool life and workpiece surface quality (Figure 10F). The presence of Ti also reflects superior coating integrity after micro-sandblasting, as the process reduces coating porosity and droplet defects, thereby enhancing coating adhesion and wear resistance. Droplets, formed by metal splashing during deposition, are weakly bonded defects with hardness similar to the substrate, which accelerate wear and promote Fe diffusion into the coating, causing diffusion wear. Area 4 exhibited coexisting oxidative, diffusion, and adhesive wear. The detection of minor Al and Ti indicates partial coating penetration and reduced protective function. Area 5 contained abundant Ni, Fe, and O, signifying severe adhesive and oxidative wear under the applied cutting conditions (Figure 10G).

4. Conclusions

In this study, the effects of micro-sandblasting pretreatment parameters on the surface properties and cutting performance of AlTiSiN-coated cemented carbide tools were systematically investigated through a combination of orthogonal experimental design and milling tests on 316 L stainless steel. The results demonstrated that micro-sandblasting pretreatment effectively modified the substrate surface morphology, improved surface uniformity, and enhanced mechanical interlocking between the coating and substrate, thereby significantly increasing coating adhesion strength and nano-hardness.

Among the pretreatment parameters, gun traverse speed was identified as the most influential factor affecting coating adhesion, followed by passivation pressure and tool rotation speed. The optimal pretreatment parameters were determined as a passivation pressure of 0.1 MPa, a gun traverse speed of 80 m/min, and a tool rotation speed of 20 r/min. Cutting performance evaluations under these conditions revealed that the optimized pretreatment notably improved tool wear resistance, extended tool life, and enhanced machining stability during the milling of 316 L stainless steel. Additionally, adjusting the forward-to-reverse rotation time ratio to 4:3 further extended tool life and improved cutting consistency.

Detailed wear mechanism analysis indicated that AlTiSiN-coated tools experienced three distinct wear stages: initial rapid wear, stable wear, and accelerated wear. The formation of a protective Al₂O₃ layer and the diffusion barrier effect of Si within the coating effectively delayed oxidation and adhesive wear, prolonging the stable wear phase. Micro-sandblasting pretreatment was found to further extend this phase by removing residual cobalt binder on the substrate surface, increasing WC grain exposure, and mitigating tensile residual stress, thereby enhancing the mechanical properties of the substrate and coating–substrate bonding strength.

Compared with untreated tools, micro-sandblasted tools exhibited smoother and more uniform flank wear regions, with reduced adhesive and abrasive wear tendencies. Notably, the cutting distance achieved before reaching the failure criterion (VBmax = 0.1 mm) increased from 4.25 m for untreated tools to 12.75 m for tools treated with optimal micro-sandblasting parameters.

Overall, this work presents a practical, efficient, and cost-effective micro-sandblasting pretreatment strategy that markedly improves the surface integrity, durability, and cutting performance of AlTiSiN-coated carbide tools when machining difficult-to-cut materials, offering clear guidance for industrial implementation and further process optimization. However, some limitations remain. Our ANOVA F-values fell below conventional significance thresholds due to limited error degrees of freedom, yet the substantial gains in adhesion, hardness, and tool life—validated by scratch tests and milling trials—underscore the practical robustness of the optimized parameters. While micro-sandblasting also smooths the tool edge, contributing to wear resistance, we did not perform precise edge-radius measurements or isolated edge-preparation studies. Future work will address these gaps by adding replicates to boost statistical power, employing 3D profilometry and subsurface microstructural analyses (e.g., XRD, TEM, and grain size measurement) for quantitative topography, and conducting dedicated edge-radius profiling to fully clarify the mechanistic role of edge rounding.