Diffusion Based Surface Modification
Introduction
Classification
Description
Application
Introduction
The process involves chemical interaction by diffusion of the coating
forming elements with the substrate.
Characteristics of Diffusion based Surface Modification
• Chemical interaction by diffusion of the coating forming elements
with the substrate
• The modified layer is a part of the substrate
• No-sharp interface and so, interface is stronger
Examples of Diffusion based Surface Modification
Carburizing, Nitriding, Borinizing, aluminizing, siliconizing,
chromizing
Advantages and Disadvantages
Advantages
• Cost effective
• Can be applied to any size and shape of component
• Strong interface
Disadvantages
• Not environment friendly
• Control is difficult
• Thermodynamic limitation on systems and extent of
alloying
• Kinetic limitation, the process is very slow
Schematic representation of the (a) diffusion assisted surface
modification (b) coating
Mechanism of Modification
a. Absorption of the active element on the surface of substrate
b. Diffusion and chemical reaction of the absorbed species with the substrate
Schematic representation of formation of a diffusion coating
Medium
Coating medium may be solid, liquid or gaseous.
When a solid medium is used, the process is called pack cementation.
The pack consists of three primary constituents:
(a) The element source which may be pure of in the form of alloy
(b) An activator
NH4Cl during pack aluminizing
(c) An inert filler
Alumina
Carburizing
Carburization is an austenitic thermo-chemical treatment
involving diffusional addition of interstitial alloying
element C into austenitic phase, and relies on the
subsequent transformation of austenite to martensite to
produce a high surface hardness.
Lower left hand portion of iron-carbon equilibrium diagram
Carburizing
Depth of Hardening:
There is no technical limit to the depth of hardening with
carburizing techniques, but it is not common to carburize to
depths in excess of 0.050 in.
Carburizing Time: 4 to 10 hours
Carburizing Temperature: 1750 oF (above the upper critical
temperature)
Quenching:
All of the carburizing processes (pack, gas, liquid) require
quenching from the carburizing temperature or a lower
temperature or reheating and quenching. Parts are then
tempered to the desired hardness.
Case depth vs. Carburizing time
Pack Carburizing:
In this process, the part that is to be carburized is packed in a
steel container so that it is completely surrounded by granules of
charcoal. The charcoal is treated with an activating chemical such
as Barium Carbonate (BaCO3) that promotes the formation of
Carbon Dioxide (CO2).
Carbon Monoxide reaction:
CO2 + C ---> 2 CO
Reaction of Cementite to Carbon Monoxide:
2 CO + 3 Fe --->Fe3C + CO2
Section through fully packed carburizing box
Quenching Process:
It is difficult to quench the part immediately, as the sealed
pack has to be opened and the part must be removed from
the pack. One technique that is used often is to slow cool the
entire pack and subsequently harden and temper the part
after it is removed from the sealed pack.
Depth of Hardening:
There is no technical limit to the depth of hardening with
carburizing techniques, but it is not common to carburize to
depths in excess of 0.050 in.
Carburizing Time:
4 to 10 hours
Pack Carburization
Charcoal or coke as C source
10-20 % alkali and alkaline earth metal carbonates as
Activators
Charcoal reacts with oxygen to form CO2
CO2 + C 2CO
2CO CO2 +C
BaCO3 BaO +CO2
CO2 + C 2CO
Liquid Carburization
2NaCN + O2 2NaCNO
4NaCNO 2NaCN +Na2CO3 +CO + 2Nnascent
2CO CO2 +Cnascent
Temperature: 850 – 950 OC
Gas Carburizing:
Can be done with any carbonaceous gas, such as methane,
ethane, propane, or natural gas. The advantage of this process
over pack carburizing is an improved ability to quench from the
carburizing temperature. Conveyor hearth furnaces make
quenching in a controlled atmosphere possible.
Liquid Carburizing:
Can be performed in internally or externally heated molten salt
pots. Carburizing salt contains cyanide compounds such as
sodium cyanide (NaCN). Cycle times for liquid cyaniding is
much shorter (1 to 4 hours), however, disposal of salt. (
environmental problems) and cost (safe disposal is very
expensive) are problems.
Vacuum Carburizing
Vacuum carburizing a steel is typically a four-step process:
1. Heat and soak step at carburizing temperature to ensure temperature uniformity
2. Boost step to increase carbon content of austenite
3. Diffusion step to provide gradual case-core transition
4. Quenching step. This may be carried out by direct quenching in oil or in a high-
pressure gas quenching system.
Schematic of a vacuum quenching furnace
Plasma carburizing
Nitriding
In this process, nitrogen is diffused into the surface of the steel
being treated. The reaction of nitrogen with the steel causes the
formation of very hard iron and alloy nitrogen compounds. The
resulting nitride case is harder than tool steels or carburized
steels.
Suitable for: Low carbon alloy steels containing Al, Cr, Mo, V, Ni
Nitriding temperature: 500 to 600 C.
Mechanism:
NH 3 N + 3H
Surface hardness achieved: up to 1000 VHN
Case Depth: 0.1 to 0. 6 mm
Applications: Gears, valves, cutters, sprockets, pump boring
tools, fuel-injection pump parts.
Schematic of Nitriding process
White layer thickness: 0.0003 to 0.002 in.
Surface hardness: 55 HRC to 70 HRC
Depth: 0.005 in to 0.020 in.
Carbonitriding
This process involves with the diffusion of both carbon and
nitrogen into the steel surface.The process is performed in a
gas atmosphere furnace using a carburizing gas such as
propane or methane mixed with several percent (by volume)
of ammonia. Methane or paropane serve as the source of
carbon, the ammonia serves as the source of nitrogen.
Quenching is done in a gas which is not as severe as water
quench.
Case hardnesses: HRC 60 to 65
Case depths: of 0.003 to 0.030
Carbonitriding gives less distortion than carburizing.
Temperature: 1400 oF -to 1600 oF
Siliconizing
Chromizing
Figure 1. Fe-Cr phase diagram
Aluminizing
Aluminizing
Technique: Pack aluminizing (calorizing)
Low temperature high High temperature low
activity (LTHA) activity (LTHA)
Two step process Single step process
Aluminizing – the process
Source of Al: Al powder or powder of Ni-Al, Cr-Al and Fe-Al
Activator: NH4Cl, NH4F, NaCl, NaF (1-2 %)
Filler: Alumina, silica, Kaolin
Inert gas flow is maintained during processing
Low temperature high activity (LTHA)
Aluminizing at 700-850 OC
Post aluminizing diffusion treatment at 1000 OC
Schematic representation of the evolution of coating structure in
a two step high activity aluminizing process
Microstructure of a typical two step high activity
aluminizing process
Schematic representation of the evolution of coating structure in
a low activity aluminizing process
Microstructure of a typical low activity
aluminizing coating on IN-100 superalloy
Various steps involved in a typical pack aluminizing
Microstructure of a typical Pt-modified
aluminide coating
