Final Report: Biomimetic Nanostructured Coating for Dry MachiningEPA Contract Number: EPD04044
Title: Biomimetic Nanostructured Coating for Dry Machining
Investigators: Jiang, Wenping
Small Business: NanoMech LLC
EPA Contact: Manager, SBIR Program
Project Period: March 1, 2004 through August 31, 2004
Project Amount: $70,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2004) RFA Text | Recipients Lists
Research Category: Nanotechnology , SBIR - Nanotechnology , Small Business Innovation Research (SBIR)
The goal of this research project was to create a novel nanostructured coating (cubic boron nitride [CBN]-TiN/polytetrafluoroethylene [PTFE] molybdenum disulfide [MoS2]) with surface features similar to a lotus leaf, for dry machining by a combination of electrostatic spray coating (ESC) and chemical vapor infiltration (CVI) processes. In particular, this research project has accomplished an innovative integration of hard (CBN) and lubrication phase(s) (TiN/ PTFE-MoS2) in such a way that the contact can be refreshed with lubricants as wear proceeds, efficiently reducing the friction in the contact and significantly extending tool life in machining.
Development of coatings for dry machining applications has been an active area of research. To date, most of the development effort has focused on multilayer coatings, often a soft lubricant phase deposited over a hard phase. In such a coating configuration, the lubricant phase wears out quickly, leaving the hard phase behind. In this investigation, a coating configuration with receptacles formed by protruded domes similar to a lotus leaf structure is proposed. The receptacles are ideal for nano- and microscale lubricants.
To achieve the coating proposed in this research project, a combined processing technology, consisting of ESC and CVI processes, along with a laser texturing/plasma etching process, was applied to create the patterned nanostructured hard coating, CBN-TiN, coupled with soft coating PTFE-MoS2. Specifically, the ESC process was employed to deposit CBN powder (a combination of submicron powder and micron powder) on the selected carbide tools. The CBN-deposited tool inserts then were infiltrated with TiN using the CVI process. A dispersion of nanosized MoS2 in PTFE then was deposited using ESC. Finally, laser texturing and plasma etching were carried out.
During this research project, all of the proposed tasks have been completed and the objectives have been achieved. The success of Phase I established an important technical platform for Phase II. In accordance with the goals set in Phase I, extensive explorative and characterization work has been carried out. A summary of the research activities performed in this Phase I research project follows.
Coating Design and Procurement of Coating Materials and Cutting Tools
The coating was designed with a structure of hard phase (CBN-TiN) controlled surface morphology (micron domes and reservoirs) as a base coating, and soft phases (nano and submicron MoS2 dispersed in PTFE) by following the biomimetic approach. A CBN-TiN composite coating proved to be efficient for cutting tool applications. PTFE and MoS2 are widely used solid lubricants for friction and wear reduction. The selection of cutting inserts was based on the potential market in which NanoMech, LLC, is interested, and on availability from commercial tool providers. MoSTTM-coated tool inserts were used as the benchmark for tribological testing.
Setup of the ESC System and the Optimization of the Process for Lotus Leaf Surface Morphology
An ESC system with a positioning unit was set up, as shown in Figure 1. Six combinations of submicron and micron CBN powders, together with different settings of ESC processing parameters, including electrical voltage at the electrode, electrode-to-substrate distance, and main air pressure, were studied to create the desired lotus leaf surface morphology, as shown in Figure 2(a). The deposition of first micron and then submicron CBN powders at the optimized conditions produced surface morphology similar to a lotus leaf, as shown in Figure 2(b).
Figure 1. ESC setup used in this investigation (1: powder feeder, 2: powder pump, 3: spray gun, 4: substrate holder, 5: deposition chamber, 6: recycling unit, 7: control unit).
Figure 2. Similarity of synthesized coating surface morphology to lotus leaf: (a) surface morphology of a lotus leaf, (b) surface morphology of ESC deposited CBN particles of various size, and (c) coating surface morphology after CVI.
CVI of TiN To Form Functional Coating Retaining the Features of Lotus Leaf
TiN as a binder phase was introduced by this process to ensure the functionality of the coating, while retaining the lotus leaf features. Process parameters included temperature, temperature distribution along the reaction tube, and gas pressure (partial pressure of gas components). In this investigation, H2, N2, Ar, and HCl inside the tube, as well as linear velocity, affect not only adhesion, but also the surface morphology. The infiltration of TiN was carried out in a vertical reactor in two sequential steps, starting with slow deposition of TiN to encapsulate CBN particles, and followed by a fast deposition to fill large pores and to form a capping layer (less than 2 µm) of pure TiN. Scanning electron microscopy (SEM) (XL 30, Philips) characterization of all infiltrated cutting inserts displayed varied surface morphology. A typical surface morphology of the CVI-infiltrated cutting tool insert is shown in Figure 2(c). The obtained surface morphology bears striking similarity to lotus leaves, with statistically arranged patterns.
Ball Milling of Submicron MoS2 Nanoparticles and Dispersion in PTFE
Because of the unavailability of nanoized MoS2 particles, a Spectral-Prep 8000D mixer/mill was used to produce the nanoparticles from commercially available submicron MoS2 particles (APS ~700 nm). SEM characterization showed that the size of submicron MoS2 particles after 6 hours of dry milling was significantly reduced to about 100 nm (see Figure 3). It successfully addressed the needs of nanosized MoS2 particles in this research project. The dispersion of ball-milled MoS2 nanoparticles into PTFE was achieved by mixing MoS2 and PTFE with the assistance of an ultrasonic vibrator for 30 minutes. Uniform dispersion was obtained.
Figure 3. SEM pictures showing: (a) as-purchased submicron MoS2 particles, and (b) nanosized MoS2 particles from ball milling.
ESC Deposition of MoS2 Dispersed in PTFE and Curing of the Deposition
Deposition at different ESC processing conditions was carried out to achieve uniform and crack-free solid lubricants on hard phases with controlled surface morphology. At the optimized deposition condition, the thickness of the lubricant was approximately 12 µm. Optical characterization showed few cracks in the deposition. Curing the coating was performed in a convective furnace with three steps (i.e., drying, baking, and sintering). The drying process helped to remove water and was completed under an infrared lamp for 20 seconds. The baking proceeded in the convective furnace at 290°C, and the sintering process was achieved in a convective furnace at 380°C for varied time intervals, depending on the thickness of the lubricants. Figure 4 shows the nano PTFE-MoS2 deposited on the hard phase-coated tool inserts.
Figure 4. Picture showing nano MoS2-PTFE deposited tool inserts.
Laser Texturing and Plasma Etching
A beam mask (30.00 × 30.00 mm2) with arrays of through-holes of 250 µm in diameter and of 400 µm in hole-to-hole distance was designed and fabricated using a laser (Nd:YAG) etching process for texturing. Based on the proposal, an excimer laser with a wavelength of 248 nm was to be used to selectively remove the soft phase on the domes and reinforce the adhesion of the soft phase to the hard phase. Initial experiments indicated that PTFE is transparent to laser beams of 248 nm wavelength. A transversely excited atmospheric CO2 (10.6 µm) laser was used to create the adaptive coating. The processing parameters were as follows: energy density = 20 J/cm2; exposure = 3 pulses. In addition, an oxygen plasma etching process was applied to the PTFE-MoS2-coated tool inserts because of its readiness for processing PTFE and scaling up. The processing conditions were: excitation frequency = 13.56 MHZ; power = 100 W; pressure = 2Pa; gas flow = 8 cm3/minute; time = 5 minutes. The etched tool inserts were characterized using optical microscopy. Considering many merits (low cost, easy to process PTFE, and readiness for scale up) associated with the plasma etching process, the following characterization and tribological testing focused on plasma-etched tools.
Characterization of the Coating
The etched surface profiles were characterized using optical microscopy and SEM. Most of the PTFE was efficiently removed from the dome, leaving the soft phase residing in the “receptacles” around the dome. A profilometer (Veeco Dektak 3030) was used to measure the surface roughness around the cutting tip (nose region) of the etched tools at two random directions. The measured surface roughness values at the two randomly chosen directions were consistent, with Ra of 2.7 µm for CBN-TiN-coated tools and 2.1 µm for the etched tools.
Tribological Studies on the Coating
The CBN-TiN-coated tool insert, the PTFE-MoS2-coated and etched tool insert, and the MoSTTM-coated tool insert were tested, respectively, using a ball-on-disc tribometer under the following conditions:
- Ball-on-disc test with 52,100 hardened balls
- Environmental conditions: 62±1 percent relative humidity and 26°C
- Radius of rotation: 4.5 mm
- Normal load: 100 g
- Sliding speed: 200 rpm
- Test time: 600 seconds
The tribological test results (coefficient of friction and mass gain/loss) are shown in Table 1.
Table 1. Coefficient of Friction and Mass Gain/Loss of the Tested Tools
Clearly, the tool coated with CBN-TiN/PTFE-MoS2 followed by plasma etching demonstrated the lowest value of coefficient of friction and a modest gain of mass in the test, indicating that the developed nanostructured coating has better lubrication and resistance to wear than layer-structured MoSTTM coating. Based on the above test, NanoMech, LLC, is confident that the tools will outperform in dry machining.
Based on the extensive work carried out in Phase I, an innovative coating with surface morphology similar to lotus leaf has been successfully created by a systematic approach (materials, processing, and etching). The effective integration of the hard and soft phases can refresh the contact surface with solid lubricant consistently, helping to reduce the friction and extend tool life. Tribological testing showed the developed coating has better lubrication and wear resistance than a layer-structured MoSTTM coating. The successful production of MoS2 nanoparticles using ball milling not only addressed the needs of this feasibility study, but also represents a novel opportunity for nanostructured materials. The novel coating also can be used for other applications associated with wear and friction. This is especially true for tools for dry machining in this study.
During this research project, the technical feasibility of creating a novel nanostructured coating following a biomimetic approach for dry machining, has been successfully demonstrated. Preliminary tribological testing has indicated that the novel coating has superior lubrication (low coefficient of friction) and resistance to wear. All proposed tasks have been accomplished. In addition to the proposed tasks, an oxygen plasma etching process was applied for the processing of PTFE-MoS2 coated tools, mainly for the scale-up consideration in Phase II. The success in Phase I has solidly established the technical platform for Phase II.