Jet plating of high melting point materials

Nov. 25, 1958

JET

G. H. SMITH ET AL

PLATING OF HIGH MELTING POINT MATERIALS

2,861,900

Filed May 2, 1955

Acetylene

Oxygen

<img file="US2861900A_D0001.tif" />

Water

Water ₄

Carrying Gas , ,

Plus Powder &amp; &amp; !2 fl &amp; 20

<img file="US2861900A_D0002.tif" />

<img file="US2861900A_D0003.tif" />

inventors ^H- Smith JOHN E PELTON RICHARD C. ESCHENBACH !EY

United States Patent Office

2,861,900

Patented Nov. 25, 1958

2,861,900

JET PLATING OF HIGH MELTING POINT MATERIALS

Kenmore, N. Y., and Richard C Eschenbach and John F, Pelton, Indianapolis Ind asoi N°ew York” ^C«Wa«o_n,TcS^n

Application May 2,1955, Serial No. 505,228

Claims. (Cl. 117—105)

The.present invention- concerns the application of improved surface: coatings to objects and particularly relates spraying, .which, are especially-advantageous for applying

Forborne ffim? “^el.^{tilig point} materials to articles .ror some timemow, it has-been-common practice to thirty ? °ru^{Ot Wilh a} ‘ ^protective coating by 'sprayin» would^{a meIted}, ^and “zed material that would adhere, to a pre-cleaned surface of the object Surface-spraying has been accomplished-by introducing the coating material, usually in the form of^: a rod into anT^flbv?V^f ft^{838 bUrnC}'' ^{inorder t0} melt the material and by thereafter' projecting the melted material - against ffie surface: to : be coated by a- blast of air or inert gas XsMNwitWb ^{Spraying has been} Practiced most suceven tW m ^{J8e of low} melting-point metals, and 30 n these coatings-have not been satisfactory for many unevenlv°7/?' *7 T ^{USUally porous and} frequently unevenly distributed. -As such, they lack many of the most desired properties of a good protective coating, such _s for example, hardness for good wear resistance and ’ imperviousness for corrosion resistance.

The present invention' has'the general purpose of over, snrfiir-A .. .*' I - above of known objecti ar To 77 f “'mi °^{ng the}'more particular higher m?i;^T ? ^feas1?¹? ^and Practicable: the use of 40 ^{t ag pwnt} materials, as well as the Usual low eltmg point materials for surface coatings. To reduce porosity m sprayed surface coatings so as to obtain i? creased ..wear and corrosion resistance in such coatings. ^A“^{d prov}® ^hardness and strength characteristics by % ^W?^{er range of availab}fr coating materials.

^{tber obje}.^{cts of thls} invention are: To provide a continuous .spraying operation utilizing high thrust to ^{against the surface} to be coated, peratures Tn ^USe7 ^{fueIs with} tower flame-temperatures. To maintain the coating particles at relatively high temperatures during their travel to the surface of the workpiece without harm to the latter. And to increase the residence time of the coating material in the flame for a longer period than heretofore possible a method^ 7 ^{the present} invention,' there is provided a method of- flame spraying a surface coating on workpieces which includes bringing the coating material to a high temperature by introducing a fuel-oxygen mixture under pressure appreciably above atmospheric into a confined space where burning is initiated and introduce» coating material into the combustion-space, and there° after piopelli.ng the heated particles toward the surface of the workpiece to be coated at temperatures and linear nartkle? n^grea&lt;i^{r than 500}„ ^feet: Per ‘ second such that the ^P t t, ” ^at^{east at} itowtog temperature upon imof tto ^{y dl};?^{charglng the} burning gases through a portion to exit vPtoHi;^SA^{PaCe In}t ^wbl?^{h such} gases are accelerated ¹ 7 ^l .,greater : than' 2000 feet per second sufficiently .high to impart such linear velocities to the’ 70 particles. _; Flowing temperature” may be defined as-· that temperature, determined by summing_: both the : thermal or^dn?bv? TT °^{f the} material, at Which the material pr, at. least, the lowest melting point constituent of such material becomes plastic. Tn the use of many coating ?ouud?^Sit^{Part?CUlarly metal and metaI alIoys} and comP nds, it is important to maintain the burning gases’ composition non-oxidizing and non-decarburizing to the ^P -Th? ή^Π °7^er: to. produce a coating of desired quality.

The spraying operation is continuous and,, in the preferred practice of this invention, comprims susnUdtog made up of a fuel and. a combustion supporting agent m proportions that avoid an excessive oxidizing fnd decarburizing . atmosphere upon combustion. The combustion supporting agent may be air. but oxygen is preL?a?se^Pof7he hrh^fl^{11 Plating high meIting} materials its use ^g 4 T temperatures produced with

7777· ^{C g} material may be introduced into the combustion zone by suspension, either in the fuel or in the Sw°^{r lf may be}_c^susPended in the combustible mixture before or even afterburning is initiated. The high flame temperatures . and the (high linear velocity of the flame jet essential to successful jet flame coating is obof the tot ^PaT^S 7® Partide-carrying mixture to a burner tur? under 7 PPmim-stion type where ignition of the mixture under pressure produces large volumes of flaming ^Sfi^{SeS that are discharged} to the outsidt ? ^confimng passageway that effects acceleration i)l the gases to high velocities.

hen.?! 7^gk temperatures to which the particles can be heated by being entrained' in the combusting mixture and in the jet flame and the appreciable temperature increase fh?^r?^Ph^ndl?^1Si-^{tO kmetic energ}y expended upon impact of to b·?™ Particles upon the surface of the work to.be coated make it possible to melt even high melting pomt:matenalsFor-at .least the lowest melting point com stituent.of such materials) sufficiently to insure a ffirm mechanical bond with the surface of the body to be c°^ated· The high velocity imparted to the particles at ipon irnnfet ί^UreS m^a?^eS 7®“ ‘° ^deform sufficiently upon impact to weld together to other particles in the coating so as to form a substantially non-porous coating Moreover, in the method described abov the ffiermal that the surface of the workpiece need not be heated to weffi bondeH^atUreS’ ^{38 hard}r^facipg’ ^etc- Consequently, madice 5 ih °^n;PO7·^{8 C}°^ah?^{gS can be} Produced by the practice of -this invention without causing major microstructural changes in the workpiece

-These aiffi other/features, objects and advantages of d^b.^s„.!ⁿJ^enbon will become apparent from the following whereih: ^deSCnPtlOD °^{f &amp;}® ^accompanymg drawings^

Figure 1 shows a‘longitudinal cross-section through a 5SS5a?23 “^{me s}°° ^for *^praMte ·-* · .Figure 3 is-a longitudinal cross-section through anffifinSoS ^let -^ita^ toe Praclce of

The method of the invention will now be described in etobodimenf¹¹¹⁶?^{1011 W}‘^th ‘^K® ^drawings, which show two th? invetoton ^{of spray guns} adapted for the practice of lAlVditlUll.

ge^X^reiX^Ce-ⁿ°^ ‘° ^FigUre, ^{F a s}P^ray δ®, indicated generaily-at TO, is-shown .employing a “throat-combnc of ffi_P^bT^er -^Which-^{iS the type preferred} in ffie pmettoe the Present invention and which is similar to construction and operation to the burner disclosed in copend9mi?^{PP IC}vT¹’ ^Sehal No. 212,547. filed by George H don^th_a⁰“^^ebr?^ary 7’ !⁹⁵¹· For purposes of this invention, a throat-combustion” burner may be defined as a

- - 8,861,9 throat element constituted by a confined space unconstricted from inlet to outlet wherein a fluid combustible mixture received through such inlet at one end of the confined space is ignited within the passageway, passed through the confined space, and then discharged from 5 the confined space through such outlet at the other end of the confined space as a stream of hot burning gases to produce a flame having a high heat transfer intensity, a high velocity and substantial thrust.

For satisfactory results, it is essential that conditions 10 during combustion be maintained such that K is between 75 and 750 in the equation:

A.'P'W ₁₅ wherein: zh=cross-sectional area of said stream of fluid combustible material at the point of introduction thereof to said confined space, in square inches

A₀=cross-sectional area of said stream of burning com- 20 bustible material at the point of discharge from said confined space, in square inches

P_;=pressure at the point of introduction of said stream of fluid combustible material into said confined space, in pounds per square inch absolute 25

P₀=pressure at the point of discharge of said stream of burning combustible material from said confined space, in pounds per square inch absolute

PK=weight of fluid combustible material consumed, in pounds per second. 30

In detail, gun 10 comprises a burner 11 having a hollow cylindrical section 12 tapered at one end toward an integral elongated, centrally bored barrel 13 and open at its other end for the reception of a fuel injector 14 having a central passageway 15 axially aligned with the axis of barrel 13. A threaded mid-section of the injector engages a tapped portion 16 of section 12, the injector being held in preselected axial position in the burner by a lock nut 17.

The injector 14 is stepped at 18 to provide a mixer ⁴⁰section 19 of reduced diameter that lies in radially inwardly spaced relation to section 12 and terminates in an outlet 20 opening into the combustion throat 21 constituted by the bore of barrel 13. The mixer 19 is spaced slightly from the tapered end wall of section 12 to provide an annular passageway for fluid flow from the annular space or chamber 22 around mixer 19 into the barrel 13. A fuel feed line 24 is connected with passageway 15 through a lateral port 23, and an oxygen feed line 25 is connected with chamber 22 through a 50 lateral port 26. In order to secure flashback of the flame, which is initially ignited outside the throat, into the throat 21, the minimum diameter of the outlet 30 should not be substantially smaller than 0.02 inch.

Coating material may be introduced into the burner in comminuted form as a suspension in the fuel or in the oxygen or as a suspension in the combustible mixture. In the embodiment shown in Fig. 1, comminuted material is conveyed by a carrier gas, such as hydrogen, into a nipple 27, threadedly received into the head of a 60 centrally open adapter plug 28 that closes the rear end of passageway 15. The nipple has its rear protruding end adapted to be connected to the source of coating material and has fitted into its forward end a forwardly extending hollow stem 29 that projects into passageway 65 15 at least beyond the lateral fuel feed port 23 and delivers carrier gas and entrained coating material to mixer section 19.

Fuel and oxygen are supplied to the throat under pressure, preferably at least 15 pounds per square inch 70 gauge. As the particle-carrying fuel enters the combustion throat 21, it mixes intimately with the oxygen in the rear portion of throat 21 to form a stream of combustible mixture which starts to burn soon after mixing, producing large volumes of flaming combustion gases 75 which pass forwardly at high velocities through the unconstricted confined space of the throat and are then discharged from outlet 30 at the mouth of barrel 13 as a flame jet. The coating particles entrained in the combustion gases are then ejected from the gun in a directed flame jet having a high heat transfer intensity, a high velocity and substantial thrust. In order to prevent excessive heating of the barrel 13 during operation, a sleeve 31 is disposed around the barrel in radially outwardly spaced relation to form a water jacket 32 through which cooling water can be circulated via inlet 33 and outlet 34. _# .

Coating material may also be introduced directly into the combustion zone in powder form or in the form of a rod. The latter is illustrated in Figure 2, wherein an elongated rod 40 of solid coating material is introduced through an opening 41 at the back end of the injector 14, extends longitudinally therethrough, and projects from mixer 19 sufficiently forward into the rear portion of throat 21 that its forward tip lies in the combustion zone. The rod is moved positively by any suitable driving means, such as for example, oppositely rotating friction wheels, shown schematically at 42, which engage opposite sides of rod 40. An O-ring 43 serves to seal opening 41.

It will be seen that a spray gun employing a throat combustion type burner has the unique advantage that the path of the particles throughout its passage through the confined combustion and discharge space is not constricted and, consequently, the particles meet no obstructions upon which they might lodge and cumulatively plug the passageway.

In a modified type of spraying gun shown in Figure 3, the burner employed is of the type where combustion occurs internally, in an enclosed chamber, and the flaming combustion gases are discharged from the combustion chamber through a jet nozzle. The gun comprises a combustion chamber 50 formed within a cylindrical shell 51 and a nozzle body 52 welded to the forward end of the shell. The combustion chamber 50 threadedly receives in its rear open end an injector member 53 which has at its end remote from the combustion chamber a tapped bore 54 into which an adapter 55 is threaded. The forward portion of bore 54 is tapered convergently toward a restricted injector throat 56 which forms an entrance into the relatively large combustion chamber 50. The forward parts of the combustion chamber walls converge forwardly toward the throat 57 of a divergent discharge nozzle 58 whose exit passageway flares outwardly and forwardly.

An oxidizing agent such as gaseous oxygen with powdered coating material suspended therein is injected under pressure, preferably 15 pounds per square inch or more, through throat 56 into the combustion chamber 50 by an injector 59 which is threaded into an oxygen supply duct 49 centrally disposed in adapter 55. Injector 59 projects into the tapered portion of bore 54 in axial alignment with throat 56 and combustion chamber 50 and terminates in a frusto-conical head 60 that is spaced from tapered walls of bore 54. to provide an annular passageway 61 for fluid from bore 54 into the throat 56. A fuel such as acetylene is delivered concurrently but separately under pressure, preferably the same pressure as the oxygen, to the combustion chamber through an eccentrically arranged fuel supply duct 62 in adapter 55, the open portion of bore 54, passageway 61 and throat 56. The fuel and coating-particle-carrying oxygen mix intimately together in passing through throat 56, and the mixture under pressure burns vigorously in combustion chamber 50, producing large volumes of flaming combustion gas which flow at high velocity through the nozzle 58, carrying the coating particles with them. In this way the coating particles are entrained in the directed flame jet provided by the discharged gases, which imparts high linear velocity to the particles..

2,801,000 , J‘u^{id fuel is su}PPlied to duct 62 through a tube 63 the ηΛ,^ελί^!εη« ^fr2^{m a}-^{header 64 *} 'dirough a sleeve 65 to he adapter 55 where it registers With diict 62. The' partuht«^aJ^ry?⁸k°^{Xygen is SUpplied t0 dti}ct 49 throush a tube 66 which extends from header 64 through sleeve 65 SX T^nd,“ Walter SS with duct 49 A water jacket 67 is formed around the S^U2r of T ^{er * * * *50 a}i^{d adapter * * * * * * * * * *55 betwecn the ex}slleve 68 ihr ^^se “^ember® and an outwardly spaced sleeve 68 threaded at its rear end to sleeve 65. Coolino water, is. introduced into the jacket 67 through a ‘ Conduit 69 m header 64, sleeve 65 and duct 70 in adapter 55, for circulation through the jacket and ducts 71 in nozzle body 52 in order to cool the combustion cfiatobS the coo^tog^ZZw^eater ^{tS proVided for} withdrawing „ Ta- ^atmospbere composition, high powder velocity and high powder temperature required by the method of this invention can be obtained in the described sptoy guns ^by P^roPe^r control of the operating variables and by proper 20 proportioning of. certain parts of the gun

There are several factors that control powder temper ature. Among the more important of tfe are Σ nature of the reactants, the fuel-oxygen ratio the residence time of the coating powder in the burning gases 25 burner cooling losses, the burner to workpiece distance X^pS7^e,°X ^F.^els “^itt iX“tires, such as acetylene, for example, are desirable and where permissible, oxygen-fuel ratios that produce max’ imum flame temperatures should be used.^P Such ratios 30 Mt forth /,^USed, ^Wlth many coating materials for reasons materiX ^SUltabl® ^{for flam}e-plating ceramic as ^1^Cn/^mS ?^{owder is} dependent upon powder velocity for'the kin/ ^{flame tempe}5^{ature for} its thermal energy⁷ 35 verted ,ί n th °^{f the particles} «effectively com ^{d ther}J^nal energy upon impact on the workpiece.. This is shown in the following table of tempera S/c colSion.^PaCt’ ^{CakUlated aSSUmin8 coinpleteIy in}Temperature rise on impact

Velocity of Powder, ft./sec.

700...

.1,000.

1,450.

2,000.

3,000.

Resultant Theoretical Temperature Increase, °F.

160

320

680

1,280

2,880 50

The minimum temperature at which powder becomes plastic enough To form a good coatingwfll _of _Cou?se depend upon the material used. In any case however’ minimum temperature means the lowest permissible coat’ mg temperature of the particle at the tiw of imnac · impact temperature will be the sum of the temperature an/th ⁶/·⁸ / ^{tbe thermal} energy imparted by the flame pact ^thTh^{metIC enei}?^y °^{f the} P°wder released upon im®ϊ®^{η tbougb an} otherwise satisfactory fuel may have too low a flame temperature for low velocity processes it can now be used successfully in the presem method because of the additional heat energy addfd to oAKSr ^{impact at tbe} *^gb velocities turn affects the composition of the coating. For ex^a“iple&gt; carbon content of certain tungsten carbide powders as supplied is 4.5% to 5.0% by weight With an oxv 7n acetylene volume ratio of 1.0,⁷ carbon cotoent fo toe plating was found by combustion analysis to be abmit o’p o’ π ? ^{1-4 r}?^t10’ ^carbon content was 2.0%, and at a 2.0 ratio, it was 1.3%. Plating quality varied with the carbon content of the coating, as attested by variations 75 “iL_o^bardnes?’ ^bnllIeness and surface appearance. The effective oxidation potential, measured in this instance by -^at,¹⁰ⁿ’. ^{of vanous} combinations of fuel and the^hnt h ^clo.^{sely related} to the amount of oxidants in Sn/a T^{ng 8aSeS}· °^xidants’ ^{for exam}Pk, carbon ΐ° ,^{lde and} water, ^may he defined for purposes of this disclosure, as substances having oxidizing properties at the operating temperatures. It has been found that when applying a tungsten carbide plating, for example the fuel-oxygen ratio should be such that less than 67%’ by volume of oxidations is formed in the reaction carried ‘° ®.⁰“^pl.^et.¹⁰ⁿ’, ^{h e}‘ ^{the ratio} °f the volume of oxidants mn^^eh^Ot^ ^Iu,^{me of} P^roducts produced by the reaction to lim&gt;t^eJ^eSS .^tba? ^67%- It is particularly important to lumt the oxidizing potential of the atmosphere in the “p® _t^{f coatln}S materials readily oxidizable at high temfndm-^e7i^{SUCh metaI aDd metal carbid}e, boride, nitride and sihcide powders. It is clear that control of the com,ΈΪ,ΙΧ- “ ^ta’“····»pX ? ”^d· ''“W™. «Λ lias particularly high flame temperatures at fuel-oxygen ratios producing desirable flame compositions, has been found to be especially suitable for use in flame plating. However, other fuels which can meet the temperature and composition requirements are also suitable. For example, hydrogen, methane and ethylene have been used Sir pl.a»g method of this in“„tio„ . Powder velocity m a spray gun of the nature described is roughly proportional to gas velocity. Since the supply pressure is the primary determinant of gas velocity he pressure which can be used becomes an iilportam factor m selection of a fuel. The higher the supply ^® ®^{apable of being used} with a particular fuel e higher the attainable powder velocity. Higher powder velocities effectively add thermal energy toX rise upon release of kinetic energy on impact of the powder .against the workpiece surface. Thus, fuels permitting higher, supply pressures and higher powder velocinrartTA ^lower ?^ame temperatures and yet still be £ inv/rion ^{ΡΓ3&lt;:ίΚβ} °^{f flame PlatiDg meth}od of

The hardness and porosity of the coating are dependent to a considerable extent upon powder velocity. This Is shown m the following table obtained by spraying tungsten-carbide-cobalt alloy with a gun employing a throat combustion burner as shown in Figure 1.

Particle Velocity Peet per Second I cSti^lSoop.

Pyramid Number

Porosity

400-600_____________

600-800^: (estimated)

1,300-1,500__________

800-1,000 1,000-1,200 1,100-1,600 up to 10%. approaches 5%. less than 2%.

The advantages of high powder velocity and high temperature made available by the present inventfon me apparent whenever a non-porous, well-bonded coating is desired. These benefits are not limited to any particular

X^gHr⁶™ ’ ^f°^{r al}*°^{Ugh the} Mention is Schily adapted for coating with materials of high melting points

It IS also adapted for coating surfaces with any of a wide anety of metals, alloys, metallic compounds plastics ceramics and minerals. Base surfaces, which Say be varfatv”/ ^lnf^ny ?^{ultable manner}&gt; may also be of a wide iHuSS°^{f matenal}.^s· ^The following table shows several Sd £ ., ®^{xamples of} substances which have been usin/fiOO ^Gt^{nerally the} Platings were made using 600 cubic feet per hour of oxygen and acetylene m a burner of the type shown in Fig. 1. An oxy-acetylene ratio of 1.0 to 1.6 was employed. In the case of the copper powder, only 300 cubic feet per hour of oxygen

2,861,900 such material, and thereafter propelling heated coating particles toward the surface of the object to be coated at a linear velocity greater than 500 feet per second by discharging the particle-carrying burning gases through a confined path in which such gases are accelerated to an exit velocity sufficiently high to impart such linear velocity to the particles. . . ,

2. A method as described in claim 1, wherein said solid coating material is in comminuted form. _

3. A method as described in claim 1, wherein said solid coating material is in the form of a rod.

4. A method of applying a surface coating to an object which comprises introducing a solid coating material and a fluid combustible mixture under pressure into a confined combustion space, heating at least a portion of the coating particles to a high temperature by burning ot fluid combustible mixture in said space in the presence of such particles, and thereafter propelling heated coating particles toward the surface of the object to be coated 20 in a jet flame having a linear exit velocity greater than 2000 feet per second by discharging the particle-carrying burning gases through a confined path in which such gases are accelerated to such velocity. ,

5. A method of applying a surface coating to an object 25 which comprises mixing a fluid fuel and a combustion supporting agent to form a combustible mixture, introducing a comminuted solid coating material into said mixture introducing combustible mixture containing said comminuted solid material into a confined combustion 30 space, heating the coating particles to a high temperature by burning the fluid combustible mixture m said space in the presence of such particles, and thereafter propelling heated coating particles against the surface of the object to be coated in a jet flame having a linear exit velocity 35 greater than 2,000 feet per second by discharging the particle-carrying burning gases through a confined path in which such gases are accelerated to such velocity.

6. A method of applying a surface coating to an object which comprises mixing a fluid fuel containing, a com40 minuted solid coating material with a comblistion supporting agent to form a combustible mixture, introducing combustible mixture containing said comminuted solid material into a confined combustion space, heating the coating particles to a high temperature by burning the fluid combustible mixture in said space in the presence of such particles, and thereafter propelling heated coating particles against the surface of the object to be coated in a jet flame having a linear exit velocity greater than 2 000 feet per second by discharging the p article-carrying * - - /*&lt;.__ At- J—. nil gases are accelerated to such velocity.

7. A method of applying a surface coating to an object which comprises mixing a fluid fuel with a combustion, supporting agent containing comminuted solid coating material to form a combustible mixture, introducing combustible mixture containing said comminuted solid material into a confined combustion space, heating the coating particles to a high temperature by burning the fluid combustible mixture in said space in the presence , of such particles, and thereafter propelling heated coating particles against the surface of the object to be coated m a jet flame having a linear exit velocity greater than 2,000 feet per second by discharging the particle-carrying burning gases through a confined path in which such gases are accelerated to such velocity.

8. A method of applying a surface coating to an object which comprises mixing a fluid fuel and a combustion supporting fluid to form a combustible mixture, introducing a carrier fluid containing comminuted solid coatγη ing material into one of such fluids prior to mixing, introducing combustible mixture containing comminuted solid material into a confined combustion space, heating the coating particles to a high temperature by burning the fluid combustible in said space in the presence.of and acetylene were used. The coatings were made in the form of buttons on a flat workpiece.

powder

Plating Adherence ¹

Workpiece

Steel___________

Do.........

Do_________

Do_________

Do_________

Do_________

Do_________

Do_________

Do________

Do________

Do________

Do________

Copper-----...

Stainless Steel.

Aluminum. _--------------—

Cobalt------------------------------Copper______________________________

Iron__________________--Nickel..---------------------------Silicon (-200 mesh)----------------Silver.._______________— —

Tungsten -j-12% Co (—325 mesh)- —

Tungsten carbide +8% Co (—3 miTnngsten carbide 4-12% Ni (—10 micron). _ \,

Tungsten carbide 4-20% Ag ( 20 micron).

Chromium carbide +15% Ni (—10 micron)._

Tungsten carbide +3%ρθ----.----ζTungsten carbide.+8%. Co (—32o mesh).

Fair.

Good.

Do.

Do.

Do.

Good.

Do.

Do.

Fair.

Good.

:

i Thp rntin^s of adherence were made as fellows: I’air examination, of crⁿss-sectioned specimens showed a fissure at some point between the rfatliS anil the base metal; Good-black inclusions were observed at the baXlato? interface In cross-sectioned specimens, although otherwise adheienee seemed to be tight; Excellent-bonding of plating to base metal good, with very few or no inclusions at the inteitoce.

An example of the performance of the present invention is its capacity for depositing a substantially nonporous coating of a high melting point, abrasive-resistant hard material such as tungsten carbide compositions. Using a spray gun of the type shown in Figure 1, a tungsten-carbon-cobalt composition containing about 4/0 carbon and 9% cobalt, in a finely divided powder of the order of 325 mesh was fed into the burner of the gun at a rate of 15 pounds per hour in a hydrogen earner gas of 60 cubic feet per hour. Acetylene and oxygen at 35 p. s. i. g. were fed to the burner in a ratio of 1.4 cubic feet of the latter to 1 cubic feet of the former at a combined rate of 600 cubic feet per hour. . The workpiece, a cylindrical steel piece Vi inch in diameter and 1½ inches in length was rotated at 150 revolutions per minute and advanced Vs inch per revolution past the burner outlet, with a standoff distance of 4 inches. In this way the workpiece was coated with a thickness (on the radius) of 0.004 inch in about five seconds. The coated sample was ground and polished by known procedures to a very smooth finish, the hardness of the surface being measured at 1200 Knoop. The gun employed had a water-cooled cylindrical nozzle %2 inch inside diameter by 8 inches long. . —.-------- - The continuous nature of the present invention makes . _{s through a} confined path m which such it possible to apply a steady stream of coating particles . ...

against a surface with substantially uniform forces imparted to the particles at all times. In this way, a um. form non-porous coating can be spread over a surface in a relatively short period of operation. . ,..,

In practice, the gun may be held in either a horizontal or vertical position, and the workpiece to be coated may be moved relative to the gun or the gun may be moved relative to the workpiece. Moreover, in the plating ot certain types of work, for example plug gages, the workpiece can be held and rotated in a chuck of the lathe while the gun is moved along the length of the plug. In this way, a uniform layer would be deposited on the gage.

It will be understood that the new features of process operation and gun construction herein disclosed may be employed in ways and forms different from those of the preferred embodiments described above, without departing from the spirit and scope of the invention, as defined in the appended claims.