Thermal Behavior of Single-Crystal Diamonds Catalyzed by Titanium Alloy at Elevated Temperature

At a Glance

Metadata	Details
Publication Date	2020-07-06
Journal	Applied Sciences
Authors	Pengyu Hou, Ming Zhou, Haijun Zhang
Institutions	Harbin Institute of Technology, China Academy of Engineering Physics
Citations	6
Analysis	Full AI Review Included

Executive Summary

This study investigates the thermochemical wear mechanisms of single-crystal diamond (SCD) tools when in contact with Ti-6Al-4V alloy at elevated temperatures, simulating conditions encountered during ultra-precision machining.

Primary Wear Mechanisms: Tool wear is dominated by thermochemical reactions, specifically diffusion wear in anaerobic environments and oxidative wear in aerobic environments.
Anaerobic Wear (Argon): Titanium acts as a catalyst, causing the diamond lattice to transform into graphite (graphitization). The resulting carbon atoms diffuse massively into the Ti-6Al-4V matrix, increasing surface carbon content from 3.01 wt% to 23.31 wt%, and forming Titanium Carbide (TiC).
Aerobic Wear (Air): Oxidation is the dominant mechanism. Carbon atoms shed from the diamond react with oxygen to form gaseous CO and CO₂. The titanium alloy surface is primarily covered by stable Titanium Dioxide (TiO₂).
Temperature Dependence: Thermodynamic analysis using Gibbs free energy confirms that the solubility of carbon in titanium increases significantly with temperature, directly correlating high cutting temperatures with severe diffusion wear.
Conclusion for Machining: Reducing the temperature in the cutting zone is identified as the fundamental measure necessary to suppress the chemical wear rate of diamond tools when machining titanium alloys.

Technical Specifications

The following data summarizes the experimental parameters and key quantitative results derived from the thermal analysis experiments (TGA, EDS, XPS).

Parameter	Value	Unit	Context
Heating Temperature Range	293 to 1473	K	Range used in simultaneous thermal analysis
Heating Rate	10	K/min	Constant rate applied during thermal analysis
Gas Flow Rate (Argon/Air)	40	mL/min	Continuous flow rate of protective/reactive gas
Ti-6Al-4V Thermal Conductivity	~15	W/(m°C)	Low conductivity leads to high interface temperature
Initial Carbon Content (Ti-6Al-4V Surface)	3.01	wt%	Before thermal analysis
Final Carbon Content (Argon)	23.31	wt%	After heating in Argon (Diffusion wear)
Final Carbon Content (Air)	4.52	wt%	After heating in Air (Oxidative wear)
Excess Free Energy of Carbon (920 °C)	39.727	kJ/mol	Used for calculating carbon solubility in Ti
Carbon Solubility in Ti (298 K)	3.3817 x 10^-8	Molar Fraction (c)	Calculated minimum solubility
Carbon Solubility in Ti (1500 K)	2.37 x 10^-2	Molar Fraction (c)	Calculated maximum solubility
Key Reaction Product (Argon)	TiC, C (Graphite)	N/A	Confirmed by XPS/EDS analysis
Key Reaction Product (Air)	TiO₂, CO, CO₂	N/A	Confirmed by XPS/EDS analysis

Key Methodologies

The study employed a combination of thermal simulation and advanced surface characterization techniques to analyze the diamond-titanium interface reactions.

Simultaneous Thermal Analysis (TGA1600): Experiments were conducted using a TGA1600 simultaneous thermal analyzer, combining Differential Thermal Analysis (DTA) and Thermal Gravimetric (TG) functions.
Thermal Cycling Parameters: Diamond and Ti-6Al-4V samples were heated from 293 K to 1473 K at a controlled rate of 10 K/min under continuous gas flow (40 mL/min).
Atmosphere Variation: Two distinct environments were tested to isolate wear mechanisms:
- Argon Environment: Simulated anaerobic cutting conditions, focusing on diffusion and graphitization.
- Air Environment: Simulated aerobic cutting conditions, focusing on oxidation.
Microstructural and Elemental Analysis (SEM/EDS): Scanning Electron Microscopy (SEM) was used to observe the surface morphology of the Ti-6Al-4V sheet after heating. Energy Dispersive X-ray Spectroscopy (EDS) quantified the change in carbon and oxygen concentrations on the surface, confirming diffusion activity.
Chemical State Analysis (XPS): X-ray Photoelectron Spectroscopy (XPS) was used to determine the binding energies of Ti2p and C1s peaks. This confirmed the specific chemical compounds formed at the interface, such as TiC, TiO₂, and Ti₂O₃.
Thermodynamic Modeling: Gibbs free energy calculations (Delta G) were used to qualitatively analyze the diffusion process, predicting the solubility of carbon in alpha-titanium as a function of temperature.

Commercial Applications

The findings are critical for optimizing manufacturing processes involving difficult-to-machine alloys and high-performance cutting tools.

Ultra-Precision Machining: Direct application in the ultra-precision cutting of aerospace components (e.g., integral blade rotors, thin-wall structures) made from Ti-6Al-4V, where SCD tools are used to achieve optical-grade surface finishes.
Tool Life Enhancement: Providing the theoretical basis for developing strategies to extend diamond tool life, such as implementing cryogenic cooling or minimum quantity lubrication (MQL) to reduce cutting zone temperature below the critical diffusion threshold.
Advanced Tool Coating Development: Guiding the design of diffusion barrier coatings (e.g., ceramic or refractory metal layers) applied to diamond tools to prevent direct chemical contact and catalytic graphitization by titanium.
High-Temperature Tribology: Relevant for any industrial application involving carbon-based materials sliding against reactive metals (like Ti, Fe, Ni) under high-load and high-temperature conditions.
Process Control Systems: Establishing temperature limits and monitoring requirements for CNC machines to ensure cutting parameters do not trigger severe thermochemical wear modes.

View Original Abstract

Single-crystal diamonds are considered as the best tool material for ultra-precision machining. However, due to its low thermal conductivity, small elastic modulus and strong chemical activity, titanium alloy has poor machinability and is a typically difficult-to-machine material. Excessive tool wear prevents diamonds from cutting titanium alloy. This study conducts a series of thermal analytic experiments under conditions of different gas atmospheres in order to research the details of thermochemical wear of diamonds catalyzed by titanium alloy at elevated temperatures. Raman scattering analysis was performed to identify the transformation of the diamond crystal structure. The change in chemical composition of the work material was detected be means of energy dispersive X-ray analysis. X-ray photoelectron spectroscopy was used to confirm the resultant interfacial thermochemical reactions. The results of the study reveal the diffusion law of the single-crystal diamond under the action of titanium in the argon and air environment. From the experimental results, the product of the chemical reaction corresponding to the interface between the diamond and the titanium alloy sheet could be found. The research results provide a theoretical basis for elucidating the wear mechanism of diamond tools in the titanium alloy cutting process and for exploring the measures to suppress tool wear.

Tech Support

Original Source

DOI: https://doi.org/10.3390/app10134651

References

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