U.S. patent application number 13/643478 was filed with the patent office on 2013-02-14 for method and equipment for removal of ceramic coatings by co2 coatings.
This patent application is currently assigned to TURBOCOATING S.P.A.. The applicant listed for this patent is Bruno A. Allegrini, Carlo Giolli, Andrea Scrivani. Invention is credited to Bruno A. Allegrini, Carlo Giolli, Andrea Scrivani.
Application Number | 20130040538 13/643478 |
Document ID | / |
Family ID | 42942267 |
Filed Date | 2013-02-14 |
United States Patent
Application |
20130040538 |
Kind Code |
A1 |
Scrivani; Andrea ; et
al. |
February 14, 2013 |
METHOD AND EQUIPMENT FOR REMOVAL OF CERAMIC COATINGS BY CO2
COATINGS
Abstract
A method for removing ceramic coatings using a special equipment
without modifying the characteristics of the substrate such as
roughness and thickness and able to prepare the substrate to be
recoated with a new ceramic layer, is provided. The removing of the
ceramic coating without damaging the substrate characteristics is
obtained by a combination of coating/substrate pre-heating by
irradiation during or immediately before the stripping and improved
solid CO.sub.2 blasting equipment and parameters.
Inventors: |
Scrivani; Andrea;
(Solignano, IT) ; Giolli; Carlo; (Solignano,
IT) ; Allegrini; Bruno A.; (Osio Sotto, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Scrivani; Andrea
Giolli; Carlo
Allegrini; Bruno A. |
Solignano
Solignano
Osio Sotto |
|
IT
IT
IT |
|
|
Assignee: |
TURBOCOATING S.P.A.
Solignano
IT
|
Family ID: |
42942267 |
Appl. No.: |
13/643478 |
Filed: |
April 27, 2011 |
PCT Filed: |
April 27, 2011 |
PCT NO: |
PCT/IB2011/051839 |
371 Date: |
October 25, 2012 |
Current U.S.
Class: |
451/39 ;
451/75 |
Current CPC
Class: |
B24C 1/003 20130101;
B24C 1/08 20130101 |
Class at
Publication: |
451/39 ;
451/75 |
International
Class: |
B24C 1/00 20060101
B24C001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 29, 2010 |
IT |
PR2010A000031 |
Claims
1. A method for removing ceramic coatings (2) applied on metallic,
ceramic, plastic or composite substrates (1), comprising stripping
by blasting phase with solid CO2 (4) blasting; said method being
applied without modifying or damaging the substrate (1)
characteristics, thickness and roughness, and being able to prepare
the substrate (1) to be recoated with a new ceramic layer (2);
characterized in that provides a combination of
coating(2)/substrate(1) pre-heating by irradiation during or
immediately before said stripping by blasting with solid CO2
(4).
2. Method, as recited in claim 1, wherein said components are
pre-heated in sequence in the pre-heating stations (5) then the
coating is stripped with solid CO2 (4) up to the quenching of the
process, namely room temperature; the steps of pre-heating and
solid CO2 blasting are repeated for each substrate up to the
complete coating removal.
3. Method, as recited in claim 1, characterized in that apply a
continuous, namely not pulsed, solid CO2 blasting flow.
4. Method, as recited in claim 3, characterized in that said solid
CO2 blasting flow is applied with constant pressure.
5. Method, as recited in claim 1, characterized in that said
coating/substrate pre-heating by irradiation applies with a speed
in a range of 1.degree. C./min to 100.degree. C./min.
6. Method, as recited in claim 1, characterized in that said
coating/substrate pre-heating by irradiation is increased up to
1000.degree. C.
7. Method, as recited in claim 3, characterized in that said
continuous flow applies a mass flow of solid CO2 in a range of
about 100-3500 g/min.
8. Method, as recited in claim 3, characterized in that said
continuous flow can vary the pressure in a range of 1-30 bar.
9. Method, as recited in claim 1, characterized in that is able to
jet the solid CO.sub.2 saving the pellets density in a range of
1.525-1.6 g/cm.sup.3 before the impact on the ceramic coating.
10. Method, as recited in claim 1, characterized in that removes
ceramic coating with a speed of 1-100 cm/min.
11. Equipment for removing ceramic coatings (2) applied on
metallic, ceramic, plastic or composite substrates (1), comprising
stripping by blasting phase with solid CO2 (4) blasting;
characterized in that comprises at least a pre-heating Station
(5)and at least sand Blasting Machine Station (20) for blasting the
ceramic coatings (2) with solid CO.sub.2 and comprising at least a
compressor and at least a feeder unit in order to feed dry ice into
one or more spray guns (19), obtaining a solid CO.sub.2 blasting
flow by the combination of the application in the blasting device
of feeder and spray gun (3) two-hose nozzle.
12. Equipment, as recited in claim 11 characterized in that is made
by N pre-heating stations (5) and M sand blasting machine stations
(20), where N=1-100 and M=1-50.
13. Equipment, as recited in claim 11 characterized in that dry ice
pellets (4) are continuously dispensed in the feedstock area (12)
of the feeder unit and an air pressure flow, in a range of 1-5 bar,
moves the accumulated pellets up to two-hose nozzle of spray guns
(3); solid CO.sub.2 pellets are fed in the main nozzle hose by
axial injection, and a second pipe with high pressure air up to 30
bar is connected with the convergent/divergent nozzle so that the
high pressure air is accelerated by convergent/divergent nozzle up
to supersonic speed; the dry ice pellets are injected directly in
the accelerated high pressure air stream after the nozzle throat
(19).
14. Equipment, as recited in claim 11 characterized in that said
pre-heating station comprises infrared (IR) lamps (6) in a range of
wavelength of 1-10 .mu.m and power output in a range of 1000-50000
W.
15. Method, as recited in claim 1, characterized in that does not
include the step of pre-damaging the ceramic coating before
stripping step by dry ice blasting.
Description
BACKGROUND
[0001] The industrial market is focused on the optimization of the
products quality and on the production and environmental costs
reduction. In this contest the surface engineering has more and
more importance because it enables to get better component
performances only modifying the component surface. One of the main
aspects of the surface engineering is the application of ceramic
thick coatings and ceramic thin films. The thick coatings are
defined as the protective layer with a thickness greater than 100
.mu.m while the thin films are defined as protective layers with a
thickness lower than 100 .mu.m. Ceramic thick coatings are made by
thermal spray technologies as Air Plasma Spray (APS), Vacuum Plasma
Spray (VPS), Suspension Plasma Spray (SPS), Solution Precursor
Plasma Spray (SPPS) and High Velocity Oxygen Fuel (HVOF), mainly.
Ceramic Thin Films are applied by Chemical Vapor Deposition (CVD)
and Physical Vapor Deposition (PVD), mainly. Thick Ceramic Coatings
are used for different application: [0002] Coatings to improve
component wear resistance as Al.sub.2O.sub.3, Cr.sub.2O.sub.3,
Al.sub.2O.sub.3--TiO.sub.2, Al.sub.2O.sub.3--ZrO.sub.2--TiO.sub.2;
[0003] Coatings to improve the component corrosion resistance as
Al.sub.2O.sub.3, Al.sub.2O.sub.3--TiO.sub.2, Cr.sub.2O.sub.3,
ZrO.sub.2--CaO; [0004] Alumina-based coatings to improve electrical
insulation of a metallic component; [0005] Thermal Barrier Coatings
are composite coatings systems. TBC systems consist of (i) a Bond
Coat (BC) of MCrAlY alloy (where "M" can be Ni, Co or a combination
of both) and (ii) a ceramic Top Coat (TC) of Yttria Partially
Stabilized Zirconia (YPSZ). MCrAlY coatings are able to protect the
substrates from high temperature oxidation and hot corrosion.
Zirconia coating, for its low thermal conduction coefficient is
able to reduce the service temperature at the substrate surface in
combination with a cooling gas system. For these reasons TBC
systems are applied on gas turbine hot parts. Usually, the
specifications of the main OEMs (original equipment manufacturers)
require the deposition of MCrAlY alloys by low pressure plasma
spray (LPPS) or vacuum plasma spray (VPS). Other methods such as
air plasma spray (APS) and high velocity oxygen fuel (HVOF) may be
desirable for their lower cost. The thermally sprayed ceramic TC
adhesion is mainly determined by the BC roughness that must have an
Ra greater than 10 .mu.m (about 12-16 .mu.m) to guarantee a good
thermal fatigue resistance during the component service life.
Ceramic TC in YPSZ is applied on metallic BC by APS; [0006]
Coatings to improve bio-compatibility of the metallic prosthesis
based on TiO.sub.2 and hydrossyapatite
Ca.sub.10(PO.sub.4).sub.6(OH).sub.2.
[0007] Ceramic Thin Films are applied for different application:
[0008] Thin films to modify optical properties of the component;
[0009] Decorative thin films based of metallic nitrides and oxides;
[0010] Thin films to improve wear and corrosion resistance of the
components based on metallic nitrides, oxides and oxi-nitrides,
mainly.
[0011] The ceramic coating removal is an important aspect in the
production of coated components. "Decoating" or "stripping" is
needed during the production of new components as well as for the
reconditioning of existing ones: [0012] (i) Stripping offers the
possibility to correct problems of coating quality (thickness,
porosity, roughness, adhesion, etc.) during the production steps;
[0013] (ii) During repair operations on serviced coated components,
the removing of the ceramic layer is the first operation step;
[0014] The main characteristic of the stripping processes is to
remove the coating without damaging the substrate characteristics
(avoiding corrosion, geometrical variations, etc.). The Thermal
Barrier Coating removal is a good example to understand the
stripping process. The TC and/or BC stripping is necessary on new
coated parts during MCrAlY or TBC manufacturing to correct problems
of coating quality and during repair operations on serviced coated
components. At the state of the art the removing of the TBC system
is very time consuming and expensive: If it is necessary to remove
only the ceramic TC, it is necessary to remove all the TBC using
sand blasting to strip the ceramic top coat and chemical acid
attack to remove the metallic MCrAlY BC. This procedure is
necessary because sand blasting decreases bond coat roughness which
is fundamental for TC adhesion. So, in the end, it is necessary to
strip both the coatings to fix only the top coat. This leads to
have very high re-work and environmental costs. It would be
desirable to have available a less costly and locally applicable
process by which only the ceramic coating (i.e. Thermal Barrier
Coating) can be purposefully removed without modifying the
characteristics of the substrate such as roughness and
thickness.
[0015] From the prior art, different methods for local removal of
ceramic coatings are already known (see, for example, the
publications US-A1-2005/0126001, US-A1-2004/0244910,
WO-A1-02/103088, WO-A1-2005/083158, DE-A1-10 2004 009 757,
US-A1-2004/0115447, US-A1-2004/0256504, US-A1-2003/0100242 and
DE-B4-103 60 063). Other methods for local repair of coating
systems are known from the publications US-A1-2002/0164417,
DE-T2-601, 03 612, US-A1-2003/0101687, EP-A1-1 304 446, EP-A1-0 808
913 and US-B1-6 235,352.
[0016] The complete removal of Thermal Barrier Coating Systems, by
means of chemical methods alone, or in combination with other
methods, have been handled in a different way in the publications
DE-A1-10 2004 049 825, US-A1-2001/0009246, US-A1-2001/0009247 and
EP-B1-1 076 114.
[0017] Furthermore, it is known (Fr.-W. Bach et al., "Abtragen von
thermisch gespritzten Schichten mit dem Trockeneis-Laserstrahl",
GTS-Strahl Vol. 14, September 2004; Fr.-W. Bach et al., "Dry ice
blasting and water jet processes for the removal of thermal sprayed
coatings", Conf. Proc. ITSC 2005, Basle, p. 1542-1548 (2005)) to
remove protective coatings, such as Thermal Barrier Coatings, which
are on components, by means of a dry ice blasting process.
[0018] A possible faster and lower cost alternative to the state of
the art of ceramic coatings removal is the Dry Ice CO.sub.2
Stripping as described in a recent patent [US20080178907].
[0019] Dry-ice particle blasting is similar to sand blasting,
plastic bead blasting, or soda blasting where a media is
accelerated in a pressurized air stream (or other inert gas) to
impact the surface to be cleaned or prepared. With dry-ice
blasting, the media that impacts the surface is solid carbon
dioxide (CO.sub.2) particles. One unique aspect of using dry-ice
particles as a blast media is that the particles sublimate
(vaporize) upon impact with the surface. The combined impact energy
dissipation and extremely rapid heat transfer between the pellet
and the surface cause instantaneous sublimation of the solid
CO.sub.2 into a gas. The gas expands to nearly eight hundred times
the volume of the particle in a few milliseconds in what is
effectively a "micro-explosion" at the point of impact that aids
the coating removal process. Because of the CO.sub.2 vaporizing,
the dry-ice blasting process does not generate any secondary waste.
All that remains to be collected is the removed coating.
[0020] As with other blast media, the kinetic energy associated
with dry-ice blasting is a function of the particle mass density
and impact velocity. Since CO.sub.2 particles have a relatively low
density, the process relies on high particle velocities to achieve
the needed impact energy. The high particle velocities are the
result of supersonic propellant or air-stream velocities.
[0021] Unlike other blast media, the CO.sub.2 particles have a very
low temperature of -109.degree. F. (-78.5.degree. C.). This
inherent low temperature gives the dry-ice blasting process unique
thermodynamically induced surface mechanisms that affect the
coating or contaminate in greater or lesser degrees, depending on
coating type. Because of the temperature differential between the
dry ice particles and the surface being treated, a phenomenon known
as thermal shock can occur. As a material's temperature decreases,
it becomes brittle, enabling the particle impact to break-up the
coating and sever the chemical bond that is weakened by the lower
temperature. The thermal gradient or differential between two
dissimilar materials with different thermal expansion coefficients
can serve to break the bond between the two materials. This thermal
shock is most evident when blasting a nonmetallic coating or
contaminate bonded to a metallic substrate.
[0022] For example, in the case of TBC stripping, Dry Ice stripping
should enable the removal of ceramic TC without modifying the
MCrAlY bond coat characteristics and mainly the surface
morphology.
[0023] The state of the art of dry ice stripping developed in the
previous cited patents offers a coating removal methods which risk
to damage the substrate and suffer from low efficiency or very long
term duration.
SUMMARY OF THE INVENTION
[0024] It is an object of the invention to show the limit of the
actual applied methods to remove ceramic protective layers and to
provide a method and an equipment for removing ceramic thick and
thin coatings with high removal efficiency and without damaging the
substrate. This method does not include the step of pre-demaging
the ceramic coating before removing the ceramic layer using dry ice
blasting as included instead in the previous cited patent
[US20080178907]. The pre-damaging using shot peening or another
sand blasting method using abrasive media shows the risk to damage
the substrate characteristics as roughness and thickness.
[0025] The only pre-heating is not able to pre-damage the ceramic
coating. The authors tested different kind of pre-heating and
quenching: pre-heating from 200.degree. C. to 1000.degree. C. and
quenching in water or liquid nitrogen. The pre-heating alone or the
combination of pre-heating and quenching are not able to pre-damage
or to remove the ceramic coating as a TBC or to get faster the dry
ice stripping process. The only thermal shock is not able to remove
the ceramic coating. Dry Ice Blasting is not able to remove in a
fast way the ceramic coatings treated with shot peening or/and
pre-heating as indicate in the previous cited patent
[US20080178907].
[0026] Only a combination of a pre-heating during or immediately
before the solid CO.sub.2 blasting can lead to a fast ceramic
coating stripping. This method includes only pre-heating by
irradiation performed during or immediately before the blasting
with solid CO.sub.2. This is due to the ceramic coating damage
mechanism.
[0027] During impact of the dry ice grains, some of the kinetic
energy due to the high velocity of the CO.sub.2 pellets and thermal
energy due to the component high temperature after pre-heating is
converted into sublimation energy. The solid carbon dioxide
sublimates to gas growing the volume by a factor of up to hundreds.
The fast sublimation of the solid CO.sub.2 creates powerful shock
waves which shot the protective coating surface and creates cracks
and removes particles of the protective coating which are already
blasted off, or which have only poor adhesion to the coating.
[0028] This effect decreases in function of the coating/substrate
temperature up to the quenching. In fact starting, for example,
from room temperature, the TBC removing speed is very slow. So the
pre-damaging is not useful. The dry ice stripping method developed
in this invention takes care of all the parameters involved in the
stripping mechanism.
[0029] The density of the solid CO.sub.2 pellets is proportional to
the shock wave power. The more the pellets are dense, the more the
gas volume growing during the sublimation is greater, the more the
shock wave is stronger. The equipment used in this invention is
able to maintain high the density of the CO.sub.2 pellets sudden
out of the spray gun nozzle.
[0030] The mass flow of the solid CO.sub.2 pellets is proportional
to the shock wave power. The more the amount of Dry Ice pellets
interact with the coating surface subliming, the stronger is the
shock wave. The higher is the gas pressure transporting the solid
CO.sub.2 pellets, the higher is the removal rate. In fact the gas
pressure helps the removal of the fragments from the fragmented
ceramic coating after the impact of the shock waves. The higher is
the gas pressure transporting the solid CO.sub.2 pellets, the
higher the solid CO.sub.2 pellets mass flow can be.
[0031] At the state of the art blasting machines for dry ice can
use only a discontinuous (pulsed) solid CO.sub.2 blasting flow.
This limits the removal rate because the stripping happens only
when solid CO.sub.2 sublimation is present and it is greater if the
temperature of the substrate/coating is higher. If the mass flow is
not constant the cool air gets the substrate/coating colder without
generating the removal effect. This invention presents a sand
blasting equipment able to have a continuous (not pulsed) solid
CO.sub.2 blasting flow (constant pressure). These optimized
parameters with very fast pre-heating permit fast ceramic coating
removal without damaging the characteristics of the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] The invention is subsequently explained in more detail with
reference to exemplary embodiments in conjunction with the
drawings, in which:
[0033] FIG. 1 shows a scheme of the ceramic coating to be removed
(FIG. 1(2)) by means of blasting by solid CO.sub.2 without damaging
the substrate (FIG. 1 (1)) characteristics where the coating is
deposited. Substrate characteristics are thickness and
roughness.
[0034] FIG. 2 shows in a plurality of sub-FIGS. 2(a) and 2(b) the
two different how the pre-heating step is carried out during the
solid CO.sub.2 blasting (FIG. 2(a)) or immediately before (FIG.
2(b)); 3 schematizes the sand blasting gun during the stripping
phase spraying the dry ice pellets schematized as 4. 5 schematizes
the IR lamp used to obtain fast pre-heating of the substrate
(1)/Coating (2) using IR radiation schematized as 6.
[0035] FIG. 3 schematized the Pre-Heating and Solid CO.sub.2
Blasting Stations.
[0036] FIG. 4 schematized the Core of the Solid CO.sub.2 pellets
Feeder where stationary part are indicated by the motif 7 and the
part in movement are indicated by the motif 8.
[0037] FIG. 5 shows the Scheme of the CO.sub.2 Pellets Two-Hose
Nozzle.
DETAILED DESCRIPTION
[0038] The present invention refers to a method and an equipment to
remove ceramic protective coatings 2 (i.e. Thermal Barrier Coating
such a Yttria Partially Stabilized Zirconia-YPSZ) with high removal
efficiency and without damaging the substrate 1
characteristics.
[0039] The removing of the ceramic coating 2 without damaging the
substrate 1 characteristics is obtained by a combination of
coating/substrate pre-heating by irradiation immediately before
(FIG. 2(b)) or during the stripping (FIG. 2(a)) and improved solid
CO.sub.2 4 blasting parameters. The substrate 1 can be metallic,
ceramic, plastic or composite. The substrate characteristics not
affected by the present invention are the substrate thickness and
roughness. Substrate thickness can vary in an range of 1 .mu.m to 1
m. The substrate can be rough (Ra>9 .mu.m) or smooth (Ra<9
.mu.m).
[0040] The stripping method is a single stage process where only a
combination of a pre-heating during or immediately before the
blasting with solid CO.sub.2 4 can lead to the ceramic coating
stripping. The equipment to remove ceramic protective coatings is
divided in two parts: Pre-heating Station 5 and Sand Blasting
Machine Station 20.
[0041] This method does not include the step of pre-damaging the
ceramic coating 2 before stripping step by dry ice blasting. The
substrates 1 coated with ceramic coating are pre-heated in sequence
in the pre-heating stations 5 (FIG. 3) up to the maximum
temperature that the substrate can tolerate. The pre-heating
station 5 is able to heat the coating/substrate up to a maximum
temperature of 1000.degree. C.
[0042] When the substrate/coating systems is arrived at the
optimized temperature to obtain the maximum removing velocity, the
coated component in moved in the solid CO.sub.2 blasting station
20.
[0043] The coating stripping using solid CO.sub.2 blasting with
optimized parameters is performed up to the quenching of the
process at room temperature.
[0044] The component is then moved in another pre-heating station 5
while another hot component is moved into the stripping station 20
(FIG. 3). The steps of pre-heating and solid CO.sub.2 blasting are
repeated for each substrate up to the complete coating removal. The
equipment can be made by n (N=1-100) pre-heating stations 5 and n
sand blasting machine stations 20 (M=1-50) (FIG. 3).
[0045] The Sand Blasting Machine Station 20 for blasting by solid
CO.sub.2 4 used in the method consists of a compressor, a feeder
unit to feed dry ice into one or more spray guns 3.
[0046] There are two general classes of blast machines as
characterized by the method of transporting pellets to the nozzle:
two-hose (suction design) and single-hose (pressure design)
systems. In either system, proper selection of blast hose is
important because of the low temperatures involved and the need to
preserve particle integrity as the particles travel through the
hose. In the two-hose system, dry-ice particles are delivered and
metered by various mechanical means to the inlet end of a hose and
are drawn through the hose to the nozzle by means of vacuum
produced by an ejector-type nozzle. Inside the nozzle, a stream of
compressed air (supplied by the second hose) is sent through a
primary nozzle and expands as a high velocity jet confined inside a
mixing tube. When flow areas are properly sized, this type of
nozzle produces vacuum on the cavity around the primary jet and can
therefore drag particles up through the ice hose and into the
mixing tube where they are accelerated as the jet mixes with the
entrained air/particle mixture. The exhaust Mach number from this
type of nozzle is, in general, slightly supersonic. Advantages of
this type of system are relative simplicity and lower material
cost, along with an overall compact feeder system.
[0047] Blast machines are also differentiated into dry ice block
shaver blasters and dry-ice pellet blasters.
[0048] Pellet blast machines have a hopper that is filled with
pre-manufactured CO.sub.2 pellets.
[0049] The hopper uses mechanical agitation to move the pellets to
the bottom of the hopper and into the feeder system.
[0050] The pellets are extruded through a die plate under great
pressure.
[0051] This creates an extremely dense pellet for maximum impact
energy.
[0052] The pellets are available in several sizes ranging from
0.040 inch (1 mm) to 0.120 inch (3 mm) in diameter.
[0053] The 0.120 inch (3 mm) in diameter pellets are commercially
available.
[0054] The solid CO.sub.2 blasting machine station use a continuous
(not pulsing) solid CO.sub.2 blasting flow (constant pressure).
[0055] Said continuous flow is obtained using a core in the feeder
device (FIG. 4) in combination with a spray two-hose nozzle gun 3
(FIG. 5).
[0056] The Dry Ice is fed using a special feeding device as
schematized in FIG. 4.
[0057] The dry ice pellets contained in a box 9 are moved by a
rotating scoop 10 in a large hole 11.
[0058] Then the dry ice pellets are moved from the position 11 by a
rotating punched tool in a further hole in the position 12.
[0059] In this way the dry ice pellets 4 are continuously stocked
in the feedstock area 12.
[0060] An air pressure flow (in a range of 1-5 bar) coming from 13
moves the pellets accumulated in 12 in the direction 14 up to the
two-hose nozzle of the spray gun 3. The two-hose nozzle is
schematized in FIG. 5.
[0061] Solid CO.sub.2 pellets are fed in the main nozzle hose 17 by
axial injection 16 in the internal injector 18. A second pipe with
high pressure air up to thirty bar is connected with the
convergent/divergent nozzle 19 (FIG. 5). The high pressure air is
accelerated by convergent/divergent nozzle up to supersonic speed.
The dry ice pellets 4 are injected directly in the accelerated high
pressure air stream after the nozzle throat 19.
[0062] The continuous flow shows a mass flow of solid CO.sub.2 4 in
a range of about 100-3500 g/min and a pressure in a range of 1-30
bar.
[0063] A continuous flow of solid CO.sub.2 is very important in
order to reach very high removal rate. In fact if the flow is
pulsed not only solid CO.sub.2 will arrive on the coating surface
but cool air, too.
[0064] The cool air will decrease the substrate/coating temperature
without a contribution to the stripping process that is due to the
shock waves due to the solid CO.sub.2 sublimation. In that way the
removal rate will be less than using a continuous flow of solid
CO.sub.2. High pressure is very important to increase the mass flow
and to increase the removal rate. In fact, when the shock waves
crumble ceramic coatings, the high pressure aids ceramic fragments
removal.
[0065] The solid CO.sub.2 Pellets used for blasting have very high
density (Density 1.4-1.6 g/cm.sup.3). The CO.sub.2 pellets density
is very important because the more is the density, the more is the
shock wave power due to the solid CO.sub.2 sublimation.
[0066] The sand blasting equipment is designed to maintain the
pellets density in a range of 1.525-1.6 g/cm.sup.3 before the
impact on the ceramic coating. This is obtained using in
combination the above mentioned feeder and two-hose nozzle.
[0067] The pre-heating systems is performed by IR lamps 6 using
irradiation.
[0068] This method shows two advantages: [0069] The irradiation
with IR allows to perform the pre-heating during the solid CO.sub.2
blasting; [0070] The irradiation with IR is able to heat the
substrate/coating system up to 1000.degree. C.
[0071] The heating speed depend on the nature of the substrate and
it can be in a range of 1.degree. C./min to 100.degree. C./min. The
pre-heating systems consist of IR lamps 6 in a range of wavelength
of 1-10 .mu.m and with a power output in a range of 1000-50000 W.
The method as recited in claim 1 is able to remove a ceramic
coating with a speed of 1-100 cm.sup.2/min.
* * * * *