Surface Engineering of Metals - Principles, Equipment and Technologies Part 16 pdf

In order to obtain desired results of glow discharge nitriding, four sic process parameters are controlled: ba-- chemical composition of reactive gas from 5 to 80% H2 - rest N2, - pressu

Fig 5.32 Schematic of furnace for CARBO-JONIMP glow discharge carbonitriding

and carburizing: 1 - resistance/glow-discharge heating chamber; 2 - lock; 3 - quench vestibule (From Kowalski, S., et al [54] With permission.)

in the main stream) chemical reactions take place which have a significanteffect on the rate of formation and properties of surface layers For thatreason, selection of the chemical composition of the atmosphere, its ho-mogenization and supply to direct contact with the treated load surface isone of the fundamental problems of PACVD methods

Special mention should be made of the chamber design for processessuch as glow discharge carburizing and carbonitriding, developed by theInstitute of Precision Mechanics in Warsaw [54] It comprises two cham-bers, separated by a vacuum-tight baffle The first chamber is used forcarrying out the process of nitrocarburizing (atmosphere composed of H2+ N2 + CH4) or carburizing (H2 + CH4) under glow discharge, while thesecond is for cooling of load (Fig 5.32) The cooling chamber is equippedwith a transport mechanism and a quench tank

5.4 Glow discharge applications

5.4.1 Glow discharge nitriding

Glow discharge nitriding is a method used in thermo-chemical treatment,yielding diffusion cases of varied structure, featuring high hardness, verygood fatigue properties, good wear resistance and resistance to corrosion insome environments

In the conventional gas nitriding method, nitrogen available for diffusion

is obtained as a result of dissociation of ammonia which flows continuouslyover the load surface, heated to process temperature In glow discharge ni-triding, active nitrogen particles are obtained by ionization of the reactivegas (nitrogen or a mixture of nitrogen and hydrogen), brought about bythe effect of glow discharge During the flow of current around treated

elements of the load, a strongly ionized zone is formed, known as the thodic glow (see Fig 5.2) Ions formed in this zone, as well as high energyneutral particles, bombard the load surface, heating it to the appropriatetemperature and creating conditions for the diffusion of nitrogen into thesuperficial layer In the nitriding process, the most important role in the for-mation of the layer is played by atomic nitrogen and nitrogen ions, i.e., N+and N2+ By way of example, the composition of ions in a gas atmosphere inconditions of glow discharge is the following at 1.33 hPa pressure and 800 Vpotential between electrodes:

ca for a gas mixture of 99% N2 and 1% H2: N+ (15.1%), N2+ (40.0%), H2+(5.1%), H+ (11.7%),

- for a gas mixture of 75% N2 and 25% H2: N+(30.0%), H2+(13.0%), H+(17.0%),

- for an atmosphere of pure nitrogen: N+(16.8%), N2+(55.5%) [55]

A rise of voltage between electrodes at constant gas pressure is conductive

to an increase in the concentration of active particles, e.g., a rise of voltagefrom 400 to 100 V ensures a 10-fold increase in the concentration of atomicnitrogen [55] One of the advantages of glow discharge nitriding is the possi-bility of optimization of layer structure to obtain certain usable propertiesthrough a change of process parameters

In order to obtain desired results of glow discharge nitriding, four sic process parameters are controlled:

ba chemical composition of reactive gas (from 5 to 80% H2 - rest N2),

- pressure in the working chamber within the range of 1 to 13 hPa,

- duration of process within the range of 3 to 16 h, depending on thetype of substrate material and desired layer thickness,

- temperature of load surface, dependent primarily on type of load andits material

Glow discharge nitriding features the following advantages in parison with the conventional method of gas nitriding in an ammoniaatmosphere:

com-1 Possibility of obtaining, in a controlled manner, of four basic types ofnitrided layer structures, both on plain carbon, as well as on alloy steels (with

an alloy content of up to 10%) These are only diffusion zone, diffusion zone andthe Fe4N ( γ ’) nitride, diffusion zone with the Fe2-3(CxNy) (ε phase) carbonitride,and finally, a diffusion zone with a surface compound zone, composed ofthe γ ’ and ε phases (Fig 5.33) This forms the basis for selection of the type ofnitrided structure for given service conditions of a component;

2 Possibility of treatment of components with complex shapes, e.g., shafts;

crank-3 Reduction of process time, thanks to faster heating of load to ment temperature and activation of the environment, as well as the treatedsurface of the load;

treat-4 Possibility of control of dimensional growth of components subjected

to treatment;

5 Significant economy of electric energy because only the load is jected to heating This means that heat resistant retorts and ceramic shields

sub-Fig 5.35 Hardness distributions in glow discharge nitrided layers for different steel

grades: 1 - N135M; 2 - 1045; 3 - 30HN2MFA* (0.3%C, 0.6 to 0.9%Cr, 2 to 2.5%Ni, 0.15

to 0.3%V); 4 - M2; 5 - 40H2MF* (0.4%C, 0.5 to 0.8Mn, 1.6 to 1.9%Cr, max 0.3%Ni, 0.3%Mo, 0.2%V); 6 - 4140; 7 - N9E* (0.9%C, 0.15 to 0,3%Mn, 0.15 to 03%Si, max 0.15%Cr, 0.2%Ni) (From Kowalski, S., et al [54] With permission.)

tigue properties, suitability to service in conditions of strong dynamicloading and ductility which surpasses that of other types of nitrided lay-ers Nitrided layers with the surface phase zone feature low ductility buthigh wear resistance They are suitable for service in conditions of wear

by friction when no dynamic loading occurs [54]

Glow discharge nitriding is applied in the treatment of structural alloysteels, hot work and cold work tool steels, especially those which do notsuffer significant core hardness drop at typical nitriding temperaturesabove 500ºC, of high speed steels, e.g., M2 and 9%W grades, as well as ofsteels with special properties, such as acid-resistant, heat resistant andcreep-resistant It can also be used in the treatment of titanium and mo-lybdenum and their alloys Hence, glow discharge nitriding is finding broadapplication in extending the service life of machine components and tools,e.g., injection mold screws and cylinders, dies, gears, punches, injectionnozzles, ultrasonic wave-guides, hobs, etc For example, the life of glow dis-charge nitrided hobs was extended significantly (from 3- to 10-fold, depend-ing on steel grade) in comparison with that of same components, nitrided byother means [54, 55]

Glow discharge, ensuring homogeneity of physico-chemical conditions

at the load surface, as well as the possibility of controlling layer structure,also allows the treatment of components with big dimensions and highrequirements regarding dimensional tolerance, such as crankshafts Theheat treatment of these components is encumbered with some problems,

on account of the type of loading and the tendency to undergo sional changes caused by geometry and residual stresses induced by themanufacturing process

dimen-Fig 5.36 Examples of distribution profiles for nitrogen, titanium, carbon and oxygen

in a Ti(NCO) layer of 3 µm thickness, obtained on a prior glow discharge nitrided 1H18N9T* steel (0.1%C, 18%Cr, 9%Ni, 0.5%Ti).

Fig 5.37 Anodic polarization curves in 0.5 m NaCl solution of surface layers on Armco

iron, formed in glow discharge nitriding and carbonitriding at 580°C.

Glow discharge nitriding also finds application in the treatment of hotwork tools, among others, forging dies, punches, die-casting dies and plasticmolds Very ductile monophase layers of 6 µm thickness produced at thesurface accommodate dynamic loading occurring during forging, enhancesliding properties and reduce tendency to checking as the result of ther-mal fatigue [59]

Glow discharge nitriding is also used on a broad assortment of cuttingtools, among others on milling cutters, broaches, reamers and thread taps.The process is, for those applications, carried out at 450 to 500ºC in times up

to 30 min Their core hardness (62 to 65 HRC) does not change, while surfacehardness rises to 1100 to 1300 HV1 The hard nitrided layer significantlyraises wear resistance without causing spalling of the cutting edge, thanks togood ductility

Glow discharge nitriding is finding increasing application in industry,and research currently carried on is aimed at developing modifications of theprocess, e.g., by introducing carbonitriding, oxy-carbonitriding [56, 57], ob-taining of composite layers, e.g., a nitride layer topped by a zone composed oftitanium nitride [23] or of Ti(NCO) titanium-oxy-carbonitride (Fig 5.36)

In industrial practice, a tendency is observed to replace carburizing byglow discharge nitriding or carbonitriding which creates a possibility ofenhancing service properties of the treated components, ensuring dimen-sional accuracy and elimination of finish grinding Glow dischargecarbonitriding (Fig 5.37) is carried out in an atmosphere composed of

H2 + N2 + CH4, and oxy-carbonitriding in N2+CH4+air The utilization ofglow discharge allows a reduction of treatment time, e.g at 800 ºC it ispossible to obtain in a time of 1 h a carbonitrided layer of 1 mm thicknessand good wear resistance [56]

5.4.2 Glow discharge boriding

Formation of boride layers in a gaseous environment with the utilization

of glow discharge is a new thermo-chemical treatment method, still ried out on laboratory or semi-technical scale Nonetheless, specific prop-erties of boride layers on steels, their high hardness, significant resistance

car-to wear, good resistance car-to the action of a number of chemical agents,notably to acids [58-60], constitute a basis for research aimed at developingnew methods of obtaining such layers, methods which are economicallyjustifiable and at the same time creating possibilities of control of structure,hence, of properties Currently known and used methods, e.g., powder pack,salt bath, paste and gas, are not fully controllable

The most favorable conditions for obtaining a homogenous diffusionlayer are those associated with the gas method Gaseous boriding featuressuch advantages as: lower temperatures than in other treatment methods,possibility of forming boride layers on components with complex shapesand easier process control [61] The gas atmospheres used in such a pro-cess, however, pose hazards on account of their toxicity and strong corro-sive action against the walls of the working chamber (BCl3) or their highdegree of explosiveness (B2H6) These difficulties may be eliminated by areduction of the proportion of reactive gas in the gas mixture with H2 or

H2 with Ar, and its more efficient utilization Such conditions can be ated by lowering the pressure in the working chamber and the introduc-tion of glow discharge [9, 34, 62-64] This allows shortening of process timeand lowering of treatment temperature, as well as reduction of reactive gasconsumption The pressure applied here is within the range of 1 to 13 hPa,while the content of BCl3 vapours is from 2 to 10% by volume [8] Fig 5.38shows examples of microstructure of boride layers on different metallic sub-strates, while Fig 5.39 shows an example of microstructure and distribution

cre-of boron and iron in a biphase FeB+Fe2B boride layer on 1045 steel Boridelayers are biphase, as a rule, and comprise the borides FeB and Fe2B Theirmicrohardness oscillates within the range of 1500 to 2400 HV0.1, depending

on the phase composition and their thickness reaches 200 µm after 6 h oftreatment at 850ºC The desired goal is to obtain monophase layers compris-ing only the more ductile Fe2B phase The FeB phase, being harder and morebrittle, spalls during service and acts as abrasive medium, thus acceleratingthe process of wear [9]

Fig 5.40 Appearance of surface of boride layers obtained on 1045 steel by glow

dis-charge boriding (a) and by the low pressure gas method (b) Process parameters same

for both processes: T = 800ϒC, τ = 120 s, p = 4 hPa (H2 + 10% BCl3 vapours).

Fig 5.41 Microstructure and distribution of Ni, Fe, B and P in multicomponent boride

layers, obtained on AISI1045 grade steel at various temperatures: 923, 1023 and 1123 K.

Fig 5.42 Microstructure of composite borided layers on low carbon steel (0.18%C),

obtained by the combination of plasma spraying of a nickel-base alloy PMNi35 (C+B+Si

- 8%, Co - max 1%, Fe - 3-5%, Cr - 5-8%, Ni - balance) with the process of glow discharge boriding Shown also is distribution of elements in borided layers.charge process (Fig 5.17) Electric activation of the gaseous medium, sig-nificant development of the treated surface, thus intensification of theprocess of chemisorption of boron cause the FeB boride to be formed first,

as a rule, in an atmosphere composed of vapors of BCl3 and H2 [9] Thus,depending on the amount of boron supplied, FeB or Fe2B borides may beformed Due to the effect of cathodic sputtering, borides being formed inthe initial stages are fine grained and have a developed surface Thisfavors chemisorption of boride and its diffusion into the core of the treatedsteel After the formation of a compact layer of FeB and Fe2B borides,boron may diffuse from the surface in the direction of the core of thetreated component along boride grain boundaries [8, 76] The formation

of a boride layer on the steel surface is based predominantly on the nism of reactive diffusion [9, 65] Borides formed during the early stages

mecha-of the boriding process are clearly oriented perpendicular to the surfaceand constitute a compact, tight layer (Fig 5.17), facilitating further diffu-sion of boron along grain boundaries Research carried out in the field ofglow discharge boriding is aimed at implementation of this process in theindustry, especially as part of the formation of composite and multi-compo-nent layers Figs 5.41 and 5.42 show examples of multi-component and com-posite boride layers, formed by the combination of processes of electroless nickelplating or plasma spraying of a nickel alloy with the process of glow dischargeboriding [67, 68] These are layers featuring good corrosion resistance (Figs.5.43 and 5.44), as well as good wear resistance (Fig 5.45)

anti-abrasion and anti-corrosion layers [69-74] These are methods whichmake possible the formation of such surface layers as e.g titanium carbide(TiC) and titanium nitride (TiN), aluminum oxide (Al2O3), silicon nitride(Si3N4), as well as multi-component layers like Ti(C,N), Ti(NCO) or compositelike TiC + TiN, nitrided layer + TiN or Ti(NCO), or boride layer + TiB2 or

Fe2B +(Ni,Fe)B + TiB2

CVD methods constitute a continuation of powder pack and salt bathmethods of formation of surface layers, differing by phase structure, with

a thickness up to 15 µm In the traditional form, these are unassisted

CVD processes, carried out under atmospheric pressure (Atmospheric

Pres-sure CVD or APCVD) and Low PresPres-sure CVD or LPCVD.

These are high temperature processes, thus their utilization is rily in the case of treatment of such materials as sintered carbides or suchcomponents where in service the only important aspect is wear by fric-tion, without major dynamic loading In such conditions, these layers as-sure a significant increase in the service life of treated components [4, 72].Table 5.4 shows data concerning the extension of service life of some com-ponents, made from different materials and treated by different CVD meth-ods [72, 75] A common feature of all CVD methods is the supply of theelement constituting the layer usually in the form of a halogen, e.g., TiCl4 inthe case of TiC, TiN or Ti(C,N) layers, SiCl4 in the case of Si3N4 layers and amixture of halogens, e.g., TiCl4 + BCl3 in the case of TiB2 layers.

prima-Table 5.4

Examples of application of CVD methods and extension of life of treated components

The second component of the layer may come from the substrate, e.g.,nitrogen or carbon in the case of TiN, TiC and Ti(C,N) layers [23] or from theatmosphere, e.g., oxygen in the case of oxide layers [76, 77] TraditionalCVD processes require the use of high temperatures, necessary for the oc-currence of chemical reactions which guarantee the formation of the layers(Fig 5.46) which limits the scope of their application This is because inthe case of components exposed to dynamic loading in service, or

Glow discharge treated

components Material

Type of layer Application

Life extension factor after treatment Upsetting machine punches HSS TiC, Ti(C,N) low carbon steels 2.6 Deep extrusion rams NC10 TiC stainless steel 5 Rollers NC10 Ti(C,N) cold rolled steel 2.7 Small roller N9 TiC thin aluminum sheet 4.7 Pitch circle for external grinding

of thread tap centers NC6 TiN abrasive material 5 Masoneylen valves in coal

liquefying plants 1H18N9T

flow of suspension of coal dust

in oil at up to 30 0∫C 10

CVD method designations

Table 5.6

General characteristics of selected CVD methods in the process of titanium

nitride layer formation

A successful future and broad application is predicted for the PACVDmethod, carried out in conditions of glow discharge, without prior treat-ment (Fig 5.47) or following surface treatment (Fig 5.48) with the appli-cation of gas atmospheres containing metal-organic compounds, e.g va-pors of tetrapropyloxititanium - Ti(OC3H7)4 [8, 73, 82] or Ti[N(CH2CH3)2]4

or Ti[N(CH3)2]4 [84] This process allows the formation of nent layers of the Ti(NCO), Ti(CN) and composite layers like nitridedlayer topped by a layer of titanium nitride or titanium oxycarbonitride -Ti(NCO), as well as Ti(NCO) + TiN [8, 83] Composite layers may be formedwith the utilization of single stage processes, i.e after the finished glow

multi-compo-APCVD Atmospheric Pressure Chemical Vapour Deposition

LPCVD Low Pressure Chemical Vapour Deposition

DCPACVD Direct Current Plasma Assisted Chemical Vapour Deposition

RFPCVD Radio Frequency Plasma Chemical Vapour Deposition

MPCVD Microwave Chemical Vapour Deposition

HFCVD Hot Filament Chemical Vapour Deposition

EACVD Electron Assisted Chemical Vapour Deposition

PhACVD Photon Assisted Chemical Vapour Deposition

LCVD Laser Chemical Vapour Deposition

MOCVD Metal-Organic Chemical Vapour Deposition

PAMOCVD Plasma Assisted Metal-Organic Chemical Vapour Deposition

General designation

of method APCVD LPCVD PACVD PACVD

Method of heating resistance heating of

working chamber

resistance heating or so-called indirect heating with the utilization of glow discharge

heating by glow discharge or by glow discharge with the so-called hot anode

heating by glow discharge or by glow discharge with the so-called hot anode Process temperature 1170-1220 K 1150 K 770-820 K 720-790 K Pressure inside

working chamber atmospheric 10-500 hPa 3-13 hPa 3-10 hPa

Gas atmospheres TiCl4+H2+N2 TiCl4+H2+N2 TiCl4+H2+N2 Ti(OC3H7)4+ H2+N2

Type of layer Ti(C,N), TiN TiC, Ti(C,N), TiN

TiN, composite layer: nitrided + TiN

layers of the type: Ti(OCN) or composite: nitrided + Ti(OCN)

Fig 5.49 Concentration profiles of nitrogen, titanium, carbon and iron in Ti(NCO)

layers, obtained on Armco iron at 560ϒC (a) and 520ϒC (b) in an atmosphere posed of vapors of Ti(OC3H7)4 - H2 - N2.

com-discharge nitriding process in an atmosphere of N2+H2, the working ber is filled with vapours of TiCl4 or Ti(OC3H7)4 in a mixture with hydrogenand nitrogen, and by changing process parameters the Ti(NCO) type layer isformed in a controlled process Similarly to all glow discharge treatments, bychanging process parameters, and appropriate preparation of the treatedsurface (by e.g., cathodic sputtering) it is possible to control layer microstructure,their chemical composition and thickness (Figs 5.49 and 5.50)

cham-Such layers feature good adhesion to the substrate because nitrogen (frominitial nitriding) and carbon from the matrix actively participate in the forma-tion of titanium nitride layers or of Ti(CN), Ti(NCO) or TiC layers [7, 8, 23].They all feature high surface hardness within 1600 to 2400 HV0.05 for Ti(NCO)layers, depending on carbon and oxygen content, approximately 200 HV0.05for TiN layers and 3000 to 4000 HV0.5 for TiC layers [7]

Usable properties of these layers may be shaped by their microstructure.Better corrosion resistance and wear resistance are exhibited by layers with afine grain structure which, as has been stressed, may be formed by the ap-propriate selection of chemical composition of the reactive atmosphere andthe utilization of cathodic sputtering (Figs 5.50 and 5.51)

Fig 5.52 Linear wear of nitrided layers and of composite layers, i.e nitrided + Ti(NCO),

obtained on 1H18N9T steel, vs sliding time, under loads of 200 and 400 MPa.

Fig 5.53 Wear resistance of nitrided layers (1), Ti(NCO) layers (2), composite layers,

i.e nitrided + Ti(NCO) (3) and nitrided +TiN (4) on M2 steel, vs sliding time, under

loads of 200 and 400 MPa (From Wierzchoñ, T., et al [8] With permission from Elsevier

Science.)

Superficial layers formed by CVD methods have found broad tion in industry, primarily as anti-wear and anti-corrosion coatings Thesemethods have helped to develop the tooling industry, as well as microelec-tronics and optoelectronics Theoretical works (especially those dealing withconditions of formation of multi-component and composite layers, diamondlayers and layers of boron nitride and silicon nitride, etc.) are conducted on

applica-a broapplica-ad scapplica-ale in universities applica-and industriapplica-al reseapplica-arch centers Reseapplica-arch inthis area encompasses not only mathematical modeling of CVD processesand investigating their mechanisms, but also practical applications in whichprocess automation, as well as design of versatile equipment for CVD pro-cesses, plays a major role Orientation of chemical reactions at the substratesurface and in the gas phase, methods of their activation, character of flow

of the reactive gas and its circulation in the working chamber, includingcontrol of gas flow rate distribution, all constitute the basis for optimization

de possibility of process control and automation;

- energy and material economy of modern CVD processes, utilizing,among others, the effect of glow discharge, for reduced energy and gas con-sumption through shortening of process time, lowering of process tempera-ture and the heating of only treated surfaces;

- relatively low capital cost of equipment for CVD processes, as well astheir versatility For example, equipment for glow discharge with a hotanode can be used not only for PACVD and LPCVD processes but also fordiffusion treatment, e.g., nitriding, boriding and carbonitriding Such equip-ment is capable of producing composite layers through the combination ofvarious surface treatments in one single process, besides featuring better effi-ciency of utilization of the working space inside the chamber;

- possibility of production of layers on components with complex shapes;

- possibility of utilization of gas atmospheres containing organic pounds [8, 84, 88, 91, 92], thus eliminating halogen-bearing atmospheres,commonly used in CVD