Boron Doped Diamond for Real-Time Wireless Cutting Temperature Monitoring of Diamond Coated Carbide Tools

At a Glance

Metadata	Details
Publication Date	2021-11-30
Journal	Materials
Authors	Sérgio Pratas, Eduardo L. Silva, M.A. Neto, C.M. Fernandes, A.J.S. Fernandes
Institutions	University of Aveiro
Citations	12
Analysis	Full AI Review Included

6CCVD Technical Analysis & Documentation: Boron Doped Diamond for Real-Time Wireless Cutting Temperature Monitoring

This document analyzes the research detailing the use of Boron Doped Diamond (BDD) thin films as Negative Temperature Coefficient (NTC) thermistors for real-time wireless temperature monitoring of carbide cutting tools. The findings confirm the viability of high-performance CVD diamond coatings as integrated sensor elements in harsh industrial environments, directly aligning with 6CCVD’s core capabilities in high-purity, custom diamond fabrication.

Executive Summary

This research successfully demonstrates the integration of Boron Doped Diamond (BDD) NTC thermistors fabricated via Hot Filament Chemical Vapor Deposition (HFCVD) onto industrial cutting tools for real-time, wireless temperature monitoring.

Integrated Sensing Solution: BDD thin films were deposited directly onto an insulating substrate coupled to a diamond-coated WC-Co micro end mill, functioning as a robust, in-situ thermal sensor.
Performance in Harsh Environments: The diamond thermistor maintained operational integrity and high sensitivity across a wide range (25-400 °C), suitable for high-speed machining diagnostics (e.g., milling of Inconel 718).
NTC Characteristics: The BDD film exhibited NTC behavior, with resistance decreasing sharply from approximately 600 kΩ at 25 °C to 10 kΩ at 400 °C.
Wireless Data Transmission: Real-time temperature variation was transmitted wirelessly using a Li-Fi (LED-to-photodiode) optical communication setup, overcoming challenges associated with rotating components.
Multilayer Coatings: The base carbide tools featured a robust multilayer architecture of Nanocrystalline Diamond (NCD) and Submicrocrystalline Diamond (SMCD) to maximize adhesion, stress relaxation, and wear resistance—critical for tool longevity.
Key Advantage: This one-step approach integrates sensing capability directly into a high-performance diamond coating, improving both wear properties and heat dissipation compared to traditional embedded thermocouples or external IR systems.

Technical Specifications

Parameter	Value	Unit	Context
Thermistor Operating Range	25 - 400	°C	Range tested for NTC performance
Thermistor Resistance (25 °C)	~600	kΩ	Room temperature resistance
Thermistor Resistance (400 °C)	~10	kΩ	High temperature resistance
BDD Film Thickness	~15	µm	Approximate thickness of BDD thermistor film
Highest Sensitivity (β)	3500	K	Observed in the high temperature range (250-400 °C)
BDD Resistivity (25 °C)	~900	Ω.cm	Calculated resistivity
Boron Concentration	~10<sup>14</sup>	cm<sup>-3</sup>	Low doping level required for high resistance
Thermistor Response Delay	~1	second	Compared to IR thermographic camera
Wireless Output Sensitivity	0.84	mV/°C	Measured variation at the photodiode receiver
Machining Speed (Vc)	5.48	m/min	End milling Inconel 718
Spindle Speed (n)	30,000	rpm	High-speed machining application
Axial/Radial Depth of Cut (a<sub>p</sub>, a<sub>e</sub>)	0.03	mm	Precision finishing cut parameters

Key Methodologies

The experimental setup involved two distinct HFCVD growth processes: (1) multilayer undoped diamond coating for the carbide tool, and (2) Boron Doped Diamond (BDD) film for the thermistor element.

Carbide Tool Preconditioning (WC-7Co Substrate):
- Roughening: Murakami reagent (KOH + K3Fe(CN)6 + water) for 15 min.
- Cobalt Etching: Immersion in H2SO4 : H2O2 (1:14 ratio) for 3 s.
- Seeding: Samples seeded with diamond powder prior to coating.
Undoped Multilayer Diamond Coating (SMCD/NCD Intercalation):
- Method: Hot Filament CVD (HFCVD). Tungsten filaments (Ø=150 µm).
- Layer Structure: Intercalation of 9 layers of Nanocrystalline Diamond (NCD) and Submicrocrystalline Diamond (SMCD) for enhanced stress relaxation and adhesion.
- SMCD Parameters: Tfilament = 2300 °C, Tsubstrate = 850 °C, Gas Flow (H2/CH4): 200/4 mL/min, Pressure: 15 kPa.
- NCD Parameters: Tfilament = 2250 °C, Tsubstrate = 800 °C, Gas Flow (H2/CH4): 200/8 mL/min, Pressure: 10 kPa.
Boron Doped Diamond (BDD) Thermistor Deposition (MCD Morphology):
- Substrate: Silicon Nitride (Si3N4) insulating substrate.
- Doping Method: Argon (Ar) carrier gas bubbled through a B2O3/ethanol solution.
- Doping Ratio: Boron to Carbon (B/C) ratio of 10,000 ppm.
- Growth Parameters: Tfilament = 2300 °C, Tsubstrate = 700 °C, Gas Flow (H2/CH4/Ar): 100/4/5 mL/min, Pressure: 75 kPa.
- Morphology Rationale: Microcrystalline Diamond (MCD) morphology was chosen for the thermistor layer due to its larger crystallite size, allowing for higher, more controllable doping efficiency.
Ohmic Contact Fabrication (WC Contacts):
- Tungsten oxide (WO2) was vaporized from the HFCVD hot filaments at 1800 °C under primary vacuum (0.08 kPa).
- Subsequent introduction of H2 and CH4 reduced the WO2 to form ohmic Tungsten Carbide (WC) contacts.

6CCVD Solutions & Capabilities

6CCVD provides the specialized CVD diamond materials and engineering customization required to replicate, optimize, and expand upon the advanced integrated sensor technology demonstrated in this research.

Applicable Materials

The successful integration of the NTC thermistor depends entirely on highly controlled BDD deposition and precise control over diamond morphology (MCD for the sensor, NCD/SMCD for the coating).

Research Requirement	6CCVD Solution & Grade	Benefit for Replication
Boron Doped Diamond (BDD)	Heavy Boron Doped PCD or SCD	6CCVD offers BDD materials with controlled B/C ratios for customized resistivity (Ω.cm) and thermal sensitivity (β parameter) requirements.
Multilayer WC-Co Coating	Polycrystalline Diamond (PCD) Standard or High-Adhesion Grade	We specialize in depositing high-quality PCD layers on challenging substrates like cemented carbide (WC-Co) with tailored interfaces for maximum adhesion and stress management.
Microcrystalline Diamond (MCD)	PCD - Specific Morphology Control	6CCVD engineers precise control over growth parameters (e.g., T<sub>substrate</sub>, gas mix) to deliver specific grain sizes (NCD, SMCD, MCD) required for optimization of doping and mechanical properties.
Insulating Substrates	Custom Substrate Compatibility	We routinely coat materials such as Silicon Nitride (Si<sub>3</sub>N<sub>4</sub>) and Alumina for sensing and electronic applications.

Customization Potential for Integrated Sensing

6CCVD’s in-house capabilities directly address the complexity of integrating diamond sensing elements onto highly specialized tool geometries:

Custom Dimensions and Geometries: While this paper used micro end mills (2.5 mm shaft diameter), 6CCVD can produce custom wafers and plates up to 125mm (PCD) and manage highly specific component geometries (e.g., custom inserts, micro-tools) using advanced laser cutting services.
Precise Thickness Control: We offer unmatched control over deposition thickness, vital for thin-film sensor fabrication (SCD and PCD available from 0.1µm to 500µm). The 15 µm BDD film used here is easily within our standard thickness specification.
Advanced Metalization Services: The experiment required Tungsten Carbide (WC) ohmic contacts. 6CCVD offers internal, high-specification metalization deposition (Au, Pt, Pd, Ti, W, Cu) for contact pads, interconnects, and antenna structures required for wireless sensors and thermal management devices.
Ultra-Low Polishing: For applications demanding optimal surface finish (e.g., minimizing friction on the diamond coating), 6CCVD provides polishing down to Ra < 1nm (SCD) and Ra < 5nm (Inch-size PCD).

Engineering Support

The successful development of this integrated thermal sensor required deep expertise in CVD chemistry, materials science, and electronic characterization (NTC β determination, hopping conduction analysis).

6CCVD’s in-house PhD engineering team can assist clients with material selection, custom doping profiles, and geometric optimization for complex real-time thermal monitoring projects in high-wear, harsh environments. We offer consultation on selecting the optimal diamond morphology (NCD vs. MCD) and doping level to achieve target resistivity ranges (kΩ to MΩ) and thermal sensitivities.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Among the unique opportunities and developments that are currently being triggered by the fourth industrial revolution, developments in cutting tools have been following the trend of an ever more holistic control of manufacturing processes. Sustainable manufacturing is at the forefront of tools development, encompassing environmental, economic, and technological goals. The integrated use of sensors, data processing, and smart algorithms for fast optimization or real time adjustment of cutting processes can lead to a significant impact on productivity and energy uptake, as well as less usage of cutting fluids. Diamond is the material of choice for machining of non-ferrous alloys, composites, and ultrahard materials. While the extreme hardness, thermal conductivity, and wear resistance of CVD diamond coatings are well-known, these also exhibit highly auspicious sensing properties through doping with boron and other elements. The present study focuses on the thermal response of boron-doped diamond (BDD) coatings. BDD coatings have been shown to have a negative temperature coefficient (NTC). Several approaches have been adopted for monitoring cutting temperature, including thin film thermocouples and infrared thermography. Although these are good solutions, they can be costly and become impractical for certain finishing cutting operations, tool geometries such as rotary tools, as well as during material removal in intricate spaces. In the scope of this study, diamond/WC-Co substrates were coated with BDD by hot filament chemical vapor deposition (HFCVD). Scanning electron microscopy, Raman spectroscopy, and the van der Pauw method were used for morphological, structural, and electrical characterization, respectively. The thermal response of the thin diamond thermistors was characterized in the temperature interval of 20-400 °C. Compared to state-of-the-art temperature monitoring solutions, this is a one-step approach that improves the wear properties and heat dissipation of carbide tools while providing real-time and in-situ temperature monitoring.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma14237334

References

2006 - Heat generation and temperature prediction in metal cutting: A review and implications for high speed machining [Crossref]
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2009 - Negative Temperature Coefficient Resistance (NTCR) Ceramic Thermistors: An Industrial Perspective [Crossref]
2020 - Tough negative temperature coefficient diamond thermistors comprising tungsten carbide ohmic contacts [Crossref]
2019 - Development of a Stable High-Temperature Diamond Thermistor Using Enhanced Supporting Designs [Crossref]
1994 - Fabrication and characterization of diamond film thermistors [Crossref]