Preparation and Properties of a Composite Glass Protective Lubricating Coating for the Forging of Ti-6Al-4V Alloy

Abstract

A SiO₂-Al₂O₃-B₂O₃-CaO-MgO-Na₂O glass-based protective lubricant coating was developed for Ti-6Al-4V alloy forging, featuring a fully non-toxic formulation. The coating consisted of a composite glass matrix formed by blending two phases with distinct softening temperatures, extending its operational window to 700–950 °C. The composite glass showed initial softening at 700 °C and complete melting at 800 °C, with contact angle measurements confirming superior wettability (θ < 90°) across the forging range (800~950 °C). With an increase in temperature, the surface tension of the composite glass melt decreased, and subsequently, the wettability of the composite glass melt was significantly improved. XRD revealed that the uncoated Ti-6Al-4V formed a 22 μm thick rutile TiO₂ scale with a porous structure and interfacial cracks, while the coated sample retained an amorphous glass layer with no TiO₂. Cross-sectional SEM showed a crack-free, poreless interface with strong metallurgical bonding, in contrast to the uncoated sample’s spalled oxide layer. EDS showed minimal oxygen diffusion of the glass coating into the substrate. Ring upsetting tests showed that the coating reduced friction from 0.5–0.7 to 0.3 (50–57% decrease). Collectively, the glass protective lubricant coating showed good performance in terms of protection and lubrication.

1. Introduction

Titanium alloys have gained widespread adoption across aerospace, marine, and defense sectors, primarily attributed to their low density, high specific strength, and exceptional corrosion resistance [1,2,3,4]. Among these, the Ti-6Al-4V alloy—an archetypal α + β titanium alloy—stands as the most commonly utilized material. With a β-transus temperature of approximately 980 °C, its forging operations typically occur within the 800–950 °C range. However, this process presents significant challenges: high-temperature oxidation of Ti-6Al-4V leads to substantial metal loss and waste, while the elevated friction between the alloy and the forging die exacerbates part ejection difficulties and accelerates die wear [4,5,6,7]. These issues underscore the critical need for developing advanced protective and lubrication strategies to optimize material utilization and extend die service life during titanium alloy forging.

In early research and production, solid lubricants such as graphite and MoS₂ were often used to reduce the friction during the forging process of titanium alloys [8,9,10]. However, the lubricating effect of graphite and MoS₂ can only be demonstrated at low temperatures. When the temperature is high, graphite and MoS₂ are likely to lose their lubricating function due to oxidation [11]. Meanwhile, solid lubricants often exhibit poor high-temperature protection. In recent years, glass protective lubricant coatings have emerged as promising solutions for hot working of Ti-6Al-4V—including forging and extrusion—owing to their unique physicochemical properties [12,13]. Within the forging temperature window, glass coatings can quickly soften and form a continuous and dense film, effectively isolating the metal substrate from the oxidizing atmosphere. Concurrently, they reduce friction between the billet and die, promoting uniform metal flow and enhancing forged part quality. To date, a variety of glass protective lubricating coatings have been successfully developed, and some have even achieved industrial production, encompassing silicate [14,15,16], borate [17], and phosphate glass [18,19]. While these coatings have mitigated key challenges in titanium alloy forging, critical technical limitations persist that demand urgent resolution.

First, the oxidation of Ti-6Al-4V alloy begins at 600 °C and accelerates above 700 °C [20,21]. This requires that the glass protective lubricating coating should melt before 700 °C to form a continuous and dense protective film. This is critical for preventing oxygen diffusion into the alloy substrate. Meanwhile, within the 800–950 °C forging window, the coating must exhibit a low high-temperature viscosity to significantly reduce billet–die friction and achieve efficient lubrication. However, a single-component glass coating is difficult to simultaneously meet the dual performance requirements of protection and lubrication within such a wide temperature range. Second, existing glass lubricants often suffer from crystallization during high-temperature service, a phenomenon that triggers a dramatic increase in high-temperature viscosity, for instance, rising from 10³ to 10⁶ Pa·s within the 800–950 °C forging window. This viscosity surge not only diminishes lubrication efficiency but also accelerates mold wear due to the abrasive nature of crystalline phases [22]. Crystallization-induced structural heterogeneity further compromises the coating’s integrity, leading to interfacial delamination and reduced oxidation resistance. Third, some commercial glass lubricants contain lead or other heavy metals [23], which pose risks during manufacturing and disposal. This contradicts the growing demand for eco-friendly materials in industrial applications.

To address the critical limitations of conventional glass lubricants outlined above, this study presented a dual-component composite glass protective lubricant coating for Ti-6Al-4V forging. Composed of two glass phases with distinct softening temperatures, the coating formed a continuous protective film by 700 °C and maintained low-viscosity fluidity up to 950 °C, extending the operational temperature window by ~100 °C compared to traditional single-component borosilicate systems. The lead-free, heavy-metal-free formulation ensured environmental sustainability, while the composite architecture suppressed thermal crystallization: XRD confirmed no crystalline phase formation (which would otherwise increase viscosity) even at 950 °C, enabling sustained friction reduction. This work also introduced a novel combined evaluation method for high-temperature oxidation resistance and tribological performance under forging conditions, addressing a critical gap in existing research. Comprehensive characterization validates the coating’s dual functionality in thermal protection and lubrication.

2. Experimental Section

2.1. Glass Powder Preparation

This study employed a dual-glass composite protective lubricant coating, designated as Glass No. 1 and Glass No. 2, with compositions detailed in

Table 1. Chemical composition of glass in this study.

Types of Glass	Content (wt%)
	SiO2	Al2O3	B2O3	CaO	MgO	Na2O
Glass No. 1	30~40	2~5	25~35	5~10	5~10	10~15
Glass No. 2	60~70	10~15	5~10	5~10	0~5	5~10

. The glasses were synthesized via a conventional quenching technique. Reagent-grade starting materials—CaCO₃, Mg(HCO₃)₂, H₃BO₃, Al₂O₃, SiO₂, and NaHCO₃—were thoroughly mixed before transfer to an alundum crucible. The mixtures were heated to 1600–1700 °C in an electric furnace, held for 2 h, and then rapidly quenched in water to form glass particles. These particles were ball-milled (the rotation speed of planetary ball mill was 200 rpm) with grinding balls (Zirconia, with mixed sizes of 1 mm 3 mm and 5 mm, mass ratio 1:2:1) and ethanol in a container: the glass-to-ball mass ratio was 1:3, and the glass mass matched that of anhydrous ethanol. After 36 h of milling, the glass powder was dried and sieved through a 200-mesh screen to obtain the final product.

2.2. Preparation and Protection Performance Test of the Glass Coating

(1)

Preparation of Ti-6Al-4V alloy specimens: Cylindrical Ti-6Al-4V alloy specimens (18 mm in diameter and 5 mm in height) were first polished with 400 to 800 grit sandpaper. The polished specimens were then immersed in a 20 wt.% aqueous NaOH solution at 60 °C for decontamination. After alkali cleaning, the specimens were placed in 40 °C hot water for ultrasonic cleaning. Finally, the cleaned specimens were dried in an oven at 100 °C for subsequent use.

(2)

Preparation of glass coating: First, the as-prepared glass powder, organic binder (methylcellulose), and solvent (distilled water) were placed into a planetary ball mill jar containing mixed-size zirconia grinding balls (1 mm, 3 mm, and 5 mm; mass ratio 1:2:1). Wet ball milling was carried out at 200 rpm for 2 h to yield the desired slurry, with the component mass ratio of glass powder–methylcellulose–distilled water set at 0.75:0.75:1. The prepared slurry was then air-sprayed onto the surface of the pre-treated Ti-6Al-4V alloy substrate, followed by oven-drying.

(3)

First, both coated and uncoated Ti-6Al-4V alloy specimens were placed in a muffle furnace. The furnace temperature was then raised to 950 °C at a heating rate of 5 °C/min and held for 1 h. After cooling, specimens were removed for analysis via X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD characterized the phase compositions of oxidized specimens, while SEM with energy-dispersive spectroscopy (EDS) examined surface and cross-sectional morphologies to evaluate the glass coating’s protective efficacy.

2.3. Determination of the Wetting Angle

(1)

Preparation of the titanium alloy sample: First, the rectangular parallelepiped Ti-6Al-4V alloy sample (length: 30 mm, width: 20 mm, height: 2 mm) was polished on the sandpaper with a mesh size of 400 to 800. Then, the titanium alloy sample was cleaned and dried according to the method in Section 2.2.

(2)

Preparation of the glass powder cylinder: Glass No. 1 and Glass No. 2 glass powders were uniformly mixed in a certain proportion and then put into a mold to be pressed into a cylinder with a diameter of 10 mm and a height of 5 mm.

(3)

Heating test: The as-prepared glass powder cylinder was placed on the pre-treated Ti-6Al-4V alloy samples, which were then loaded into a muffle furnace. The furnace was heated to the target temperature (600~950 °C) at 5 °C/min and held isothermal for 15 min, and the samples were subsequently removed.

(4)

Measurement of the wetting angle: After the sample was cooled, photos were taken with the rear camera of an Apple iPhone 8 (12 MP, 4032 × 3024 pixels). Then, a tangent was drawn to the gas–liquid interface, and the wetting angle θ between the glass droplet and the surface of the titanium alloy was measured.

2.4. Determination of the Friction Coefficient

Numerous techniques exist for determining friction coefficients in metal plastic forming; this study employed the simple and reliable ring upsetting method [24]. During ring specimen upsetting, changes in inner diameter and height are measured. By comparing these dimensional variations with literature-standard friction factor curves [25], the friction coefficient of the sample can be determined. The experimental method was as follows:

(1)

Preparation of the titanium alloy sample: Ring upsetting specimens were fabricated from Ti-6Al-4V alloy. As shown in Figure 1, the ring dimensions followed an outer diameter–inner diameter–height ratio of 6:3:2. A 0.2 mm deep groove was machined on the specimen surface to retain the glass coating.

(2)

Preparation of the glass coating: The prepared glass slurry was brushed into the surface groove of the titanium alloy specimen, which was then oven-dried at 100 °C for 1 h to cure the coating.

(3)

Upsetting process: Specimens were placed in the Gleeble 3500 thermomechanical simulator mold and clamped. The system was rapidly heated to 950 °C and held for 5 min, and then axial compression was applied to induce deformation. The strain rate was set at 1/s, with a target height reduction (ΔH) of 40%. H13 tool steel was used for the upper/lower dies.

(4)

Determination of the friction coefficient: Inner diameters were measured before and after deformation to calculate the diameter change (ΔD). Combining ΔD with the height reduction (ΔH), the friction coefficient (µ) was derived from the standard curve (Figure 2) [24].

2.5. Analytical Characterization

The glass transition (T_g) temperatures of the glass were determined by differential scanning calorimetry (DSC, NETZSCH) in air, with a heating rate of 10 °C/min. The Tg value in this study was determined using the tangent method, a classic thermomechanical analysis approach shown in reference [26]. The surface and cross-sectional morphology and element distribution of specimens were analyzed with a scanning electron microscope (SEM, ZEISS from Oberkochen, Germany) and an Oxford EDS spectrometer. The phase compositions of the specimens were observed with an X-ray diffraction instrument (XRD, PANalytical from Almelo, The Netherlands), with 1.5418 Å Cu ka radiation.

3. Results and Discussion

3.1. Determination of the Glass Proportion in the Composite Coating

The composition of the glass protective lubricant coating was systematically designed to meet the dual functional requirements of oxidation resistance and lubrication during Ti-6Al-4V forging. Given that the oxidation of Ti-6Al-4V initiates at 600 °C and accelerates above 700 °C, the coating must melt below 700 °C to form a dense, continuous film that blocks oxygen diffusion to the substrate. Additionally, within the forging temperature range (800–950 °C), the coating must exhibit low viscosity to minimize friction between the alloy and die. To achieve these requirements across a broad thermal window, a composite glass approach was adopted.

The composite glass consisted of two complementary components. The first component—characterized by high B₂O₃ and Na₂O content (Glass No. 1)—facilitated rapid melting at lower temperatures (softening at 700 °C) and enhanced wetting on the titanium alloy substrate. The second component—enriched in SiO₂, Al₂O₃, and MgO (Glass No. 2)—yielded elevated high-temperature viscosity and thermal stability, ensuring that the composite retained integrity up to 950 °C. By blending these glasses with distinct rheological properties, the composite achieved optimal viscosity profiles for both protective coating formation and lubrication.

Notably, both glass components were formulated without toxic constituents such as PbO. During slurry preparation, non-toxic methyl cellulose and distilled water were used as the binder and solvent, respectively, aligning with environmental sustainability requirements. This design strategy yields a high-performance, eco-friendly coating system tailored to the demanding conditions of Ti-6Al-4V hot forging.

Figure 3 presents the DSC profiles of Glass No. 1 and Glass No. 2, revealing transition temperatures (Tg) of approximately 540 °C and 670 °C, respectively. To develop an optimal glass protective lubricant for Ti-6Al-4V forging, composite coatings with varying ratios of Glass No. 1 to Glass No. 2 (detailed in

Table 2. Three kinds of coatings with different ratios between Glass No. 1 and Glass No. 2.

Types of Coating	Content of Glass No. 1 (wt%)	Content of Glass No. 2 (wt%)
Coating A	80	20
Coating B	50	50
Coating C	20	80

) were applied to the Ti-6Al-4V substrates. These samples underwent firing trials across the alloy’s forging temperature window (800–950 °C, with 950 °C representing the initial forging temperature and 800 °C the final stage). The coating surface morphology was systematically evaluated to determine the ideal composition ratio for maintaining integrity and lubricity throughout the forging process.

The surface characteristics of three glass lubricant coatings (A, B, and C) were evaluated after heat treatment at 800 °C, 850 °C, 900 °C, and 950 °C for 1 h, with the following observations:

Coating A: At 800–850 °C, the coating fully covered the titanium alloy surface, exhibiting distinct glassy luster and a smooth, flat morphology. However, at 900 °C, the coating began to flow off the substrate.

Coatings B and C: Across 800–950 °C, both coatings maintained complete surface coverage, displaying prominent glassy luster and uniform smoothness.

For Ti-6Al-4V forging applications, an ideal glass lubricant must maintain low high-temperature viscosity within 800–950 °C while resisting flow-off from the substrate. Based on these firing trials, Coating B emerged as the optimal candidate. Formulated with a 1:1 ratio of Glass No. 1 to Glass No. 2, this composition balanced viscosity and adhesion, preventing thermal flow while ensuring effective lubrication. Subsequent investigations were therefore focused on this 1:1 composite formulation.

3.2. The High-Temperature Softening and Wetting Properties of Glass

Figure 4 shows the high-temperature softening and wetting behavior of the above composite glass (the proportion of Glass No. 1 and Glass No. 2 was 1:1) within the temperature range of 700~950 °C. At 700 °C, the sample exhibited a small amount of liquid phase, densified, and showed a tendency to round, indicating initial softening of the composite glass. By 750 °C, the sample’s upper surface had become smooth and rounded, reflecting increased liquid-phase content and more pronounced softening. At 800 °C, the specimen spread uniformly on the Ti-6Al-4V substrate, confirming the complete melting of the composite glass. At the maximum service temperature of 950 °C, the glass liquid height decreased significantly due to its low viscosity and excellent fluidity. Temperatures exceeding 950 °C risked flow-off of the coating from the titanium alloy surface due to excessive viscosity reduction. These observations confirmed that within the Ti-6Al-4V forging range (800–950 °C), the composite glass maintained low viscosity, enabling optimal lubrication performance.

Meanwhile, it could be observed that within the forging temperature range (800~950 °C), the wetting angle θ of the glass melt on the Ti-6Al-4V alloy was less than 90°. The wetting behavior of glass melt on the titanium alloy surface can be classified into three processes according to the magnitude of the wetting angle θ.

(a)

Spreading: when θ = 0°, the glass melt completely spread over the titanium alloy surface.

(b)

Wetting: when θ < 90°, the glass melt adhered to the surface with partial coverage.

(c)

Non-wetting: when θ > 90°, the glass melt formed bead-like droplets and barely adhered to the surface.

So, in this study, the composite glass coating showed good wetting performance on Ti-6Al-4V alloy. Moreover, with the increase in temperature, the wetting performance gradually increased because the wetting angle θ of the glass melt on the Ti-6Al-4V alloy gradually became smaller.

The surface tensions between solid–gas, liquid–gas, and liquid–solid directly determine the wetting behavior of glass melt on the titanium alloy surface. The relationship between these surface tensions and the wetting angle θ can be described by Young’s equation, as shown in Equation (1) [27].

In this equation,

cos θ = (γ_gs − γ_ls)/γ_gl

(1)

• γ_gs is the surface tension of the solid;
• γ_ls is the interfacial tension between solid and liquid;
• γ_gl is the surface tension of the liquid.

The wetting angle θ in the above equation represents the strength of the liquid’s wetting ability on the solid surface; the smaller the θ, the stronger the wetting ability of the liquid, and vice versa [27]. It can be seen from Equation (1) that improving the wettability of the glass coating on titanium alloy can be achieved through three main approaches: first, reducing the surface tension γ_gl of the glass melt; second, decreasing the interfacial tension γ_ls between solid and liquid; and third, increasing the surface tension γ_gs of the solid. The interfacial tension γ_ls is mainly related to the compositions of the liquid and solid phases. When the compositions of the solid and liquid phases are closer, γ_ls becomes smaller, but this is generally difficult to achieve. Additionally, the surface tension γ_gs of the solid is also difficult to change under normal circumstances. Therefore, the most feasible method to improve the wetting performance of glass melt on titanium alloy is to reduce the surface tension of the glass melt.

Relevant studies have shown that the surface tension of glass melt is closely related to its composition, and this relationship can be derived from Equation (2) [27].

γ = ∑γ_in_i/n_i

(2)

Here,

• γ is the surface tension of the glass melt;
• n_i is the molar percentage of component i in the glass melt;
• γ_i is the surface tension of component i in the glass melt.

Table 3. Surface tension of oxides in glass.

Composition	Surface Tension Value (σ × 10−3 n/m)
SiO2	290
A12O3	380
MgO	520
CaO	510
BaO	470
TiO2	250
Na2O	250
K2O	Variable; very small; may be negative
B2O3	Variable; very small; may be negative

[27] presents the surface tension values of various oxides comprising the glass matrix. Oxides such as TiO₂, Na₂O, K₂O, and B₂O₃ exhibited relatively low surface tension values. In this study, the composite glass composition incorporated elevated levels of B₂O₃ and a controlled amount of Na₂O. The inclusion of these oxides significantly reduced the surface tension of the glass melt, thereby enhancing its wettability on the titanium alloy substrate. As the temperature increased, the surface tension of the glass melt further decreased, leading to a reduction in contact angle and a corresponding improvement in wetting performance.

3.3. Protective Performance of the Coating

Figure 5 displays the XRD pattern of the oxide scale formed on uncoated Ti-6Al-4V after oxidation at 950 °C for 1 h. The results revealed that the primary oxidation product was rutile-type TiO₂ (PDF#21-1276). An amorphous peak was observed at approximately 25 °, though it is critical to note that the oxide scale naturally spalled from the Ti-6Al-4V substrate post-oxidation. The analysis was performed on exfoliated oxides placed on a glass slide (a silicate glass, inherently amorphous), meaning the amorphous peak at ~25 ° originated from the glass slide substrate rather than the oxidation products themselves. As shown in Figure 6a, the oxide primarily consisted of TiO₂ in varying morphologies with a porous structure. Figure 6b indicates that the TiO₂ oxide scale measured approximately 22 μm in thickness, with a distinct crack forming between the Ti-6Al-4V alloy substrate and the oxide scale.

After oxidizing the glass-coated Ti-6Al-4V sample at 950 °C for 1 h, the coating remained fully intact on the substrate without any loss or delamination. The XRD pattern (Figure 5) confirmed the glass coating retained its amorphous structure post-oxidation, with no detectable TiO₂ formation. Notably, the coating resisted thermal crystallization even after prolonged holding at 950 °C, critical for forging applications, as crystallization would increase viscosity and degrade lubrication performance.

Figure 7 presents cross-sectional SEM images and EDS line scans of the oxidized coating. The microstructure clearly showed the glass coating maintaining robust bonding to the Ti-6Al-4V substrate after oxidation and mechanical preparation. The interface exhibited no pores or cracks, demonstrating strong adhesive integrity. EDS analysis revealed minimal oxygen diffusion into the substrate, with the coating acting as an effective diffusion barrier. Collectively, XRD (amorphous structure), cross-sectional SEM (intact interface), and EDS (limited oxygen penetration) confirm that the composite glass coating provides exceptional high-temperature oxidation protection, leveraging its amorphous morphology and defect-free bonding to suppress oxygen ingress.

To further characterize the coating’s protective performance, the glass-coated Ti-6Al-4V specimens were heat-treated at 600 °C, 700 °C, 800 °C, and 950 °C for 1 h, with surface morphologies analyzed by SEM. Figure 8 presents SEM micrographs of the coating at different temperatures. At room temperature, the coating consisted of particles (1.5 ± 0.3 μm average diameter, quantified via ImageJ (version 1.8.0) analysis of multiple fields) interspersed with varying-size pores.

Heating to 600 °C initiated particle fusion, reducing porosity. By 700 °C, increased melting resulted in a dense coating surface with only isolated open pores, critical because Ti-6Al-4V oxidation accelerates at this temperature, and the dense coating effectively blocked oxygen ingress. At 800 °C, the coating exhibited full vitrification, forming a smooth, pore-free barrier. At the maximum forging temperature of 950 °C, the coating surface remained extremely flat, providing an impermeable oxygen barrier. These observations demonstrate the coating’s temperature-dependent densification mechanism, which progressively enhances oxidation resistance across the forging temperature range.

3.4. Lubricating Performance of the Coating

As shown in

Table 4. Friction coefficient of samples.

Sample	The Amount of Height Reduction ΔH	The Amount of Inner Diameter Reduction ΔD	The Friction Coefficient µ
Ti-6Al-4V alloy without glass coating	40%	22%	0.5~0.7
Ti-6Al-4V alloy with glass coating	40%	8%	0.3

, applying the glass protective lubricant coating to the titanium alloy surface reduced the friction coefficient by nearly 50%, demonstrating the coating’s effective lubrication performance. Notably, the measured friction coefficient (0.3) was higher than the values (≈0.1) reported in the literature [28,29], likely attributable to experimental method differences. In this study, friction coefficients were measured using a Gleeble 3500 system manufactured by Dynamic Systems Inc. (DSI) from New York, NY, USA, where specimens were resistance-heated via current adjustment with thermocouple feedback. Since the glass coating was non-conductive, the titanium alloy only made contact with the dies at a thin edge, leading to uneven specimen heating. In areas with poor die–specimen contact, lower temperatures prevented complete glass coating melting, thereby increasing frictional resistance.

To address this discrepancy and validate the coating performance more comprehensively, we plan to conduct future ring compression tests using traditional forging presses, which better simulate industrial conditions and eliminate the conductive heating limitations. Additionally, we will perform a post-test surface characterization (e.g., SEM/EDS) to analyze wear patterns and lubricant distribution, which will be incorporated into our ongoing research.

4. Conclusions

This study reports the development of a composite glass protective lubricant coating for Ti-6Al-4V forging. The coating formulation was free of toxic components like PbO, using non-toxic methyl cellulose as the binder and distilled water as the solvent, fully meeting environmental safety standards. Composed of two glass phases with distinct softening temperatures, the composite architecture extended the coating’s operational window: it initiated softening at 700 °C and achieved complete melting by 800 °C. Wettability measurements confirmed that across the Ti-6Al-4V forging range (800–950 °C), the composite glass melt exhibited optimal wetting on the titanium substrate. As the temperature increased, the melt’s surface tension gradually decreased, enhancing wetting performance. Protection tests demonstrated that the coating effectively inhibited high-temperature oxidation of Ti-6Al-4V. Critically, the glass coating resisted thermal crystallization even after prolonged holding at 950 °C—a key advantage, as crystallization would increase viscosity and degrade lubrication efficiency. Ring upsetting tests validated that the coating reduced the friction coefficient from 0.5~0.7 to 0.3, demonstrating its dual functionality of high-temperature oxidation protection and efficient lubrication for Ti-6Al-4V forging applications.