Process for the formation of wear- and scuff-resistant carbon coatings

At a Glance

Metadata	Details
Publication Date	2023-01-23
Journal	Lincoln (University of Nebraska)
Authors	Gerard W. Malaczynski, Xiaohong Qiu, J. V. Mantese, Alaa A. Elmoursi, Aboud H Hamdi
Institutions	Cranbrook Academy of Art, Northern New Mexico College
Analysis	Full AI Review Included

Executive Summary

This analysis details a highly efficient Plasma Source Ion Implantation (PSII) process for forming adherent, wear- and scuff-resistant carbon coatings, specifically targeting complex, three-dimensional aluminum alloy components (e.g., A390 pistons).

Core Objective: To enable the use of light, soft aluminum alloys in high-wear applications (like engine blocks/pistons) by applying a hard, low-friction surface coating, eliminating the need for heavy iron liners.
Adhesion Mechanism: The process utilizes carbon ion implantation (up to 40 kV) to form intermetallic carbides (aluminum carbide, silicon carbide) in a shallow subsurface layer (75-150 nm), ensuring superior coating adherence.
Multi-Step Process: The method involves four distinct, sequential plasma immersion steps: (1) Argon sputter cleaning (oxide removal), (2) Carbon ion implantation (carbide formation), (3) Argon sputter removal (graphitic layer removal), and (4) Amorphous hydrogen-containing carbon (DLC) deposition.
Efficiency: The PSII technique allows for simultaneous, uniform treatment of all unmasked surfaces on complex geometries, making it suitable for high-volume automotive manufacturing.
Coating Characteristics: The final coating is an amorphous, hydrogen-containing carbon (often referred to as diamond-like carbon, DLC), exhibiting high hardness, wear resistance, and lubricity (low friction).
Final Thickness: The deposited DLC layer typically ranges from 1 µm to 10 µm.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	A390 Aluminum Alloy	N/A	Hypereutectic casting alloy (17% Si, 4.5% Cu)
Implantation Depth (Carbon)	75 to 150	nm	Depth of carbide formation layer
Implantation Dose (Carbon)	2 to 5 x 10¹⁷	ion/cm²	Required dose for carbide formation
Final Coating Thickness (DLC)	1 to 10	µm	Range for adherent, hard carbon layer
Preferred Coating Thickness	5	µm	Example thickness achieved in bench tests
Sputter Cleaning Pressure (Ar)	0.2 to 2	mtorr	Range for oxide removal
Preferred Sputter Pressure (Ar)	0.4	mtorr	Preferred operating pressure
Sputter Voltage (Ar)	1 to 10	kV	Negative pulse potential for sputtering
Example Sputter Voltage	1.2	kV	Example voltage for Ar ion energy
Implantation Pressure (CH₄/C₂H₂)	0.2 to 6	mtorr	Range for carbon ion implantation
Implantation Voltage	5 to 40	kV	Negative pulse potential for carbon implantation
Example Implantation Voltage	20	kV	Example voltage used for carbon implantation
Deposition Pressure (C₂H₂)	2 to 70	mtorr	Range for final DLC deposition
Example Deposition Pressure	4.5	mtorr	Example pressure for acetylene deposition
Deposition Voltage	600	V	Negative pulse potential for final deposition
Pulse Duration (Implantation)	10 to 30	µs	Short duration pulses
Pulse Duration (Deposition)	30	µs	Example duration for deposition
Duty Cycle (Sputter/Deposition)	20 to 95	percent	High duty cycle for non-implantation steps
RF Self-Bias (Alternative)	500 to 2000	V	DC self-bias for sputtering/deposition

Key Methodologies

The process utilizes a Plasma Source Ion Implantation (PSII) apparatus (System 10) involving a vacuum vessel (12) and a high voltage pulser (43) to treat workpieces (30) in four sequential steps without removal from the chamber.

Surface Cleaning (Oxon Removal):
- Gas: Argon (Ar).
- Pressure: Maintained at 0.2 to 2 mtorr (e.g., 0.4 mtorr).
- Action: High voltage pulser applies negative pulses (1 to 10 kV, e.g., 1.2 kV) at several kHz.
- Result: Positive Ar ions bombard the surface, sputtering off oxygen atoms and surface oxides (SiO₂, Al₂O₃).
Carbon Ion Implantation (Carbide Formation):
- Gas: Methane (CH₄) and/or Acetylene (C₂H₂).
- Pressure: Maintained at 0.2 to 6 mtorr.
- Action: High voltage pulser applies negative pulses (5 to 40 kV, e.g., 20 kV) for 10-30 µs duration.
- Result: Carbon/hydrocarbon ions are implanted 75-150 nm deep, forming adherent aluminum and silicon carbides (AlC, SiC). A thin, extraneous graphitic layer is coincidentally deposited.
Graphitic Layer Removal (Sputter Etch):
- Gas: Argon (Ar).
- Pressure: Maintained at 0.2 to 2 mtorr (e.g., 0.4 mtorr).
- Action: High voltage pulser applies negative pulses (1 to 10 kV, e.g., 1.2 kV).
- Result: Ar ions bombard the surface, sputtering and removing the non-adherent graphitic carbon layer, exposing the carbide-containing surface.
Amorphous Carbon Deposition (DLC Coating):
- Gas: Acetylene (C₂H₂) and/or Methane (CH₄).
- Pressure: Maintained at 2 to 70 mtorr (e.g., 4.5 mtorr).
- Action: Negative electrical pulses (e.g., 600 V) are applied with a high duty cycle (20% to 95%).
- Result: An adherent, hard, amorphous hydrogen-containing carbon layer (DLC) is deposited to a thickness of 1-10 µm.

Commercial Applications

The technology is primarily designed for improving the tribological performance of light alloys in demanding mechanical environments.

Automotive Powertrain:
- Aluminum pistons operating against aluminum cylinder walls (eliminating heavy iron liners).
- Engine and transmission components subjected to sliding contact and scuffing (e.g., valve train parts).
High-Wear Machinery:
- Any sliding or reciprocating components made of aluminum, ferrous metal alloys, or other materials capable of forming intermetallic carbides.
Aerospace and Defense:
- Components requiring high surface hardness and low weight, where light metal alloys are critical.
Tooling and Molds:
- Application of DLC coatings to improve the life and release properties of complex, three-dimensional tools.
General Engineering:
- Any application requiring an adherent, low-friction coating on complex geometries where traditional line-of-sight deposition methods are impractical or inefficient.

View Original Abstract

A process for forming an adherent diamond-like carbon coating on a workpiece of suitable material such as an aluminum alloy is disclosed. The workpiece is successively immersed in different plasma atmospheres and subjected to short duration, high voltage, negative electrical potential pulses or constant negative electrical potentials or the like so as to clean the surface of oxygen atoms, implant carbon atoms into the surface of the alloy to form carbide compounds while codepositing a carbonaceous layer on the surface, bombard and remove the carbonaceous layer, and to thereafter deposit a generally amorphous hydrogen-containing carbon layer on the surface of the article.

Tech Support

Original Source

DOI: None