JP2009264132A - Engine valve and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an engine valve and a manufacturing method therefor, especially a sintered titanium-based engine valve provided with a stem portion having excellent fatigue resistance characteristic and high wear resistance and a face portion having excellent creep characteristic. <P>SOLUTION: In this engine valve, the stem portion is composed of sintered titanium-based composite material with sintered titanium-based heat-resisting alloy used as a matrix and reinforcement particle dispersively retained in the matrix, and the face portion is composed of sintered titanium-based heat-resisting alloy containing no reinforcement particle. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an automobile engine valve and a manufacturing method thereof, and more particularly to a sintered titanium-based engine valve and a manufacturing method thereof.

  Conventionally, engine performance (higher output, lower fuel consumption, lower noise) has been achieved by using a titanium alloy that is lightweight and strong for automobile engine valves. However, as titanium alloys are applied to higher performance engines, the high temperature strength of titanium alloys has increased due to problems such as an increase in load due to an increase in inertia weight accompanying an increase in engine speed and a decrease in strength due to a rise in temperature. Further improvement in fatigue strength and the like has been demanded.

In order to meet these demands, the present inventors have already proposed a titanium material excellent in hot workability, strength, creep characteristics, fatigue characteristics and wear resistance, and an engine valve using this titanium material (Patent Document 1). reference). That is, an engine valve using a titanium-based composite material comprising a matrix mainly composed of a titanium alloy and titanium compound particles and rare earth compound particles dispersed in the matrix. This engine valve is excellent in high-temperature strength, creep characteristics, fatigue characteristics, and wear resistance characteristics, because the thermodynamically stable titanium compound particles and rare earth compound particles are dispersed and held in the matrix.
International Publication No. 00/05425

  However, in the case of an exhaust engine valve, the face part including the highest temperature part may be exposed to a high temperature of 900 ° C. In the engine valve using the above titanium-based composite material, the creep characteristics of the face part are not necessarily limited. It was not satisfactory.

  The present invention has been made to solve this problem, and is a sintered titanium-based engine valve comprising a stem portion having high fatigue resistance and wear resistance and a face portion having high creep characteristics. And a manufacturing method thereof.

  The inventors of the present invention have conducted extensive experiments on a sintered titanium-based composite material having a matrix mainly composed of a titanium alloy and reinforcing particles such as titanium boride particles dispersed and held in the matrix as follows. I got a good knowledge.

  1) The sintered titanium-based heat-resistant alloy greatly improves its fatigue strength by including TiB as reinforcing particles. FIG. 4 shows the fatigue characteristics of a Ti-6Al-4V sintered alloy A (●) and a titanium-based composite material B (■) further containing 10% by volume of TiB in the sintered alloy A by a rotating bending fatigue test. It is a graph to show. It can be seen that the fatigue strength is greatly improved by including TiB.

  2) The creep characteristics of the sintered titanium-based heat-resistant alloy decrease with an increase in the amount of TiB particles that are reinforcing particles. FIG. 5 shows a Ti-6Al-4V sintered alloy A (Δ), a titanium-based composite material C (◯) containing 5% by volume of TiB particles in the sintered alloy A, and 10% by volume of TiB particles. The creep characteristics of the titanium-based composite material B (●) and the molten forging material D (▲) of Ti-6Al-2Sn-4Zr-2Mo-0.2Si are shown. FIG. 4 shows a result of a creep test in which a bending stress of 50 MPa was applied to a sample heated to a temperature of 800 ° C. in dry air, and the creep deflection with respect to elapsed time was measured. The amount of TiB particles as reinforcing particles It shows that the creep characteristics decrease with increasing.

  That is, it was found that the sintered titanium-based heat-resistant alloy improves the fatigue strength by compounding TiB particles, but decreases the creep characteristics.

  Therefore, a sintered titanium-based composite material containing reinforcing particles is used for the stem part, which must take into account strength characteristics such as bending fatigue strength and wear with the valve guide, and an exhaust gas whose maximum temperature part is 900 ° C. or higher. If a sintered titanium-based heat-resistant alloy that does not contain reinforcing particles is used for the face portion that may be exposed to the above, an excellent engine valve that satisfies the characteristics required for each portion can be obtained.

  In other words, the engine valve of the present invention is an engine valve in which a stem portion and a face portion are integrally formed, and the stem portion has a sintered titanium-based heat-resistant alloy as a matrix and dispersed reinforcing particles in the matrix. The face portion is made of a sintered titanium-base heat-resistant alloy that does not contain the reinforcing particles. Here, it is preferable that the stem portion has a needle-like (α + β) structure with a β particle size of 25 to 100 μm, and the face portion has a coarse needle-like (α + β) structure with a β particle size of 100 to 1000 μm.

  The engine valve manufacturing method of the present invention includes a material powder preparation step in which a raw material powder is mixed to prepare a material powder, a powder filling step in which the prepared material powder is filled in a cavity of a mold, and the filled A billet forming step in which material powder is pressed to form a cylindrical billet, a billet sintering step in which the formed billet is sintered to form a sintered billet, and the sintered billet is hot worked to obtain a stem A hot working process for forming a rough engine valve material including a head part and a face part, and a heat treatment process for heating the rough engine valve material to coarsen the structure of the face part. A region corresponding to the face portion of the cavity is filled with a first material powder comprising titanium powder and an alloy element powder forming a matrix together with the titanium powder, and the stem of the cavity Characterized by filling a second material powder containing reinforcing particles are further dispersed and held in the matrix material powder said first region corresponding to the.

  In the engine valve manufacturing method of the present invention, the heat treatment step is preferably a step of heating the engine valve crude material at 1100 to 1200 ° C. for 0.5 to 2 hours in an inert gas atmosphere.

  The engine valve of the present invention is an engine valve in which a stem portion and a face portion are integrally formed by sintering a titanium-based heat-resistant alloy as a matrix, and the stem portion includes reinforcing particles made of a titanium compound or the like in the matrix. It is made of a sintered titanium matrix composite that is dispersed and held. Therefore, the stem has excellent heat resistance as well as excellent fatigue strength and wear resistance, and the face consists of a heat-resistant titanium-based alloy (matrix) that does not contain reinforcing particles. With special characteristics.

  A preferred embodiment of the present invention will be described in detail below.

[I] Engine Valve As shown in FIG. 1, the engine valve 40 of the present invention includes a stem portion 110 having an annular cotter groove 111 formed at one end, and a face portion 150 connected to the stem portion 110. The stem portion 110 and the face portion 150 are formed of powder materials having different compositions.

  The stem portion 110 uses a sintered titanium-based heat-resistant alloy (hereinafter also simply referred to as a titanium alloy) as a matrix, and a sintered titanium-based composite material (hereinafter simply referred to as a titanium-based composite material) in which the reinforcing particles 200 are dispersed and held in the matrix. ). On the other hand, the face portion 150 is made of a titanium alloy that does not include the reinforcing particles 200.

  Here, the titanium alloy is not particularly limited as long as it is a titanium alloy containing a plurality of elements of at least Al, V, Sn, Zr, Si, O, Nb, and Mo, and a known one can be used as appropriate. . Among them, in mass%, Al: 3.0 to 7.0%, Sn: 2.0 to 6.0%, Zr: 2.0 to 6.0%, Mo: 0.5 to 4.0%, A titanium alloy composed of the balance Ti and inevitable impurities is preferable. The titanium alloy further contains one or more of Si: 0.1 to 0.4%, O: 0.1 to 0.5%, and Nb: 0.5 to 4.0%. May be.

  Since the titanium alloy having such a composition has excellent high-temperature strength and creep characteristics, it is suitable as a material for the face portion of an engine valve that is exposed to high temperatures.

  The stem portion requires strength characteristics such as bending fatigue strength and wear resistance against the valve guide. For this reason, the stem portion is made of the above titanium alloy as a matrix, and a titanium matrix composite material in which reinforcing particles are dispersed and held in the matrix.

Here, the reinforcing particles are preferably titanium compound particles and / or rare earth compound particles, and the titanium compound particles are titanium boride (TiB, TiB ₂ ), titanium carbide (TiC, Ti ₂ C), titanium nitride (TiN). ) And titanium silicide (TiSi ₂ ), and rare earth compound particles are oxides of yttrium (Y), cerium (Ce), lanthanum (La), erbium (Er), and neodymium (Nd) And particles composed of at least one of sulfides. These compound particles may be used alone or in combination as reinforcing particles in the stem portion.

  Such reinforced particles are compounds that exist stably up to high temperatures in titanium alloys, improve the hot workability by suppressing the grain growth of the β phase of the titanium alloy, and also at high temperatures and high temperatures. Fatigue properties and wear resistance can be improved.

  Such reinforcing particles are preferably contained in an amount of 1 to 15% by volume in the titanium-based composite material forming the stem portion. When the content of reinforcing particles is less than 1% by volume, sufficient fatigue properties or wear resistance cannot be imparted to the stem portion. On the other hand, if the content exceeds 15% by volume, the toughness of the stem portion may deteriorate, which is not preferable.

[II] Manufacturing Method of Engine Valve The engine valve of the present invention includes (1) a material powder preparation step, (2) a billet forming step, (3) a billet sintering step, (4) a hot working step, and (5) a heat treatment. It can be obtained through a process. Hereafter, the manufacturing method of this invention is demonstrated in detail along each process.

(1) Material powder preparation process A material powder preparation process is a process of mixing material powder and preparing material powder so that it may become a desired composition. First, titanium powder, alloy element powder, and reinforcing particle powder are prepared.

(1a) Titanium powder As the titanium powder, powders such as sponge titanium powder, hydrodehydrogenated titanium powder, titanium hydride powder, and atomized powder can be used. The shape and particle size (particle size distribution) of the constituent particles of the titanium powder are not particularly limited, but from the viewpoint of cost and compactness of the sintered body, the average particle size of the titanium powder is 100 to 200 μm. It is desirable to be.

(1b) Alloy element powder The alloy element powder is a powder necessary for obtaining a titanium alloy which is a main component of the matrix. Since the titanium alloy contains Al, V, Sn, Zr, Si, O, Nb, Mo and the like in addition to titanium, the alloy element powder may be a powder of each single metal, It is good also as the powder of the alloy and compound which are made by 1 type or combination of each element. Moreover, it is good also as an alloy containing all these elements. In addition, what is necessary is just to prepare the composition of alloy element powder suitably according to a composition and compounding quantity of a matrix.

(1c) Reinforced Particle Powder Reinforced particle powder is necessary to form titanium compound particles and rare earth compound particles that are dispersed in a matrix and reinforce the stem portion. The reinforcing particle powder may be a powder of a titanium compound or a rare earth compound itself. Further, it may be boron, carbon, nitrogen, silicon or the like, rare earth element simple substance, alloy or compound powder that reacts with the matrix element elements (titanium, oxygen, etc.) to form titanium compound particles or rare earth compound particles. Here, examples of titanium compound particles include titanium boride particles (TiB, TiB ₂ ), titanium carbide particles (TiC, Ti ₂ C), titanium nitride particles (TiN), or titanium silicide particles (TiSi ₂ ). Can do. The titanium compound particles may be a combination thereof as well as one of these. Of these, titanium boride particles (TiB, TiB ₂ ) are preferable.

  Examples of rare earth compound particles include oxides or sulfides of yttrium (Y), cerium (Ce), lanthanum (La), erbium (Er), and neodymium (Nd). The rare earth compound particles may be not only one of these but also a combination thereof. Alternatively, the above-described titanium compound particle powder and rare earth compound particle powder may be combined to form a reinforced particle powder.

(1d) Preparation of material powder The following two types of material powders are prepared using the titanium powder, alloy element powder and reinforcing particle powder prepared as described above. That is, a first material powder in which titanium powder and alloy element powder are blended to have a predetermined composition, and a second material powder in which a predetermined amount of reinforcing particle powder is blended with the first material powder. . In addition, what is necessary is just to use well-known mixing means, such as a V-type mixer, a ball mill, and a vibration mill, for mixing of raw material powder.

(2) Billet forming step The billet forming step is a step in which the material powder obtained in the material powder preparation step is filled in a cylindrical cavity of a mold and pressed to form a cylindrical billet. The billet formed here is formed into an engine valve rough material 30 having a stem portion 11 and a face portion 15 shown in FIG. Therefore, in this billet molding process, first, the first material powder prepared in the material powder preparation process corresponds to the face layer 10a of the billet molded body 10 (FIG. 2a) that becomes the face section 15 after hot working. The cylindrical cavity is filled with the second material powder, and then the cavity is filled with the second material powder so as to correspond to the stem portion layer 10b of the billet molded body 10 which becomes the stem portion 11 after hot working. That is, the filling is performed so that the second material powder is laminated on the first material powder in the cavity. And these material powder with which the cavity was filled is pressurized with a lamination | stacking state, and the billet molded object 10 is formed. In addition, there is no restriction | limiting in particular in the pressure molding method, What is necessary is just to carry out similarly to the past.

(3) Billet sintering step The billet sintering step is a step in which the billet 10 obtained in the billet forming step is sintered at a temperature equal to or higher than the β transformation point of the matrix and cooled to room temperature to obtain a sintered billet. By this billet sintering step, the powder particles in contact with each other in the billet compact are sintered.

When the billet compact 10 is heated to the β transformation point or higher, the titanium powder and the alloy element powder are alloyed in the face layer 10a of the billet compact 10 to form a titanium alloy as a matrix. At the same time when forming the same titanium alloy and the stem portion layer 10b in the face portion layer 10a of the billet molding 10, if reinforcing particles is TiB ₂ is new with the titanium powder (such as TiB) Particles are formed. That is, by sintering, a titanium-based composite material in which reinforcing particles are dispersed in a matrix mainly composed of a titanium alloy is formed in the stem portion layer 10b.

  The sintering in the billet sintering step is preferably performed in a vacuum or an inert gas atmosphere. Although sintering temperature is performed in the temperature range more than (beta) transformation point, the temperature range is more preferable in it being 1200 to 1400 degreeC. The sintering time is preferably 2 to 16 hours. In sintering below 1200 ° C., densification is not always sufficient. On the other hand, sintering at a temperature exceeding 1400 ° C. is uneconomical in energy and not efficient from the viewpoint of productivity. Further, if the sintering time is less than 2 hours, it may not be sufficiently densified, and sintering for 16 hours or more is uneconomical.

  Immediately after the sintering, the face layer 10a of the billet 10 is in a β single phase. When this is cooled at a cooling rate of 0.1 to 10 ° C./s, a needle-like α phase is precipitated from the β phase and a mixed phase (α + β phase) of the β phase and the α phase is formed. The strength of the matrix is remarkably improved by the precipitation strengthening of α phase and coarse needle-like formation, and the creep characteristics in the high temperature region of the face portion are improved.

  On the other hand, the stem portion layer 10b has a two-phase structure of a titanium alloy β phase and TiB particles (titanium compound particles) immediately after the end of sintering. A phase precipitates to form a mixed phase of β phase, acicular α phase and TiB particles. This mixed layer improves the wear resistance and fatigue characteristics of the stem portion. In addition, this TiB particle | grains suppress effectively the coarsening of (beta) phase grain, when a stem part is hot-worked.

(4) Hot working process The hot working process is a process in which the sintered billet 10 is hot worked to form the engine valve rough material 30 in which the face portion 15 is continuously provided on one end side of the stem portion 11. The sintered billet 10 obtained in the billet sintering step forms a mixed phase (α + β phase) of a β phase and a needle-like α phase in the face portion layer 10a, and in the α + β phase in the stem portion layer 10b. Thus, a mixed phase in which reinforcing particles such as TiB particles are dispersed is formed. For this reason, even if the sintered billet is hot worked at a temperature equal to or higher than the α + β region or the β transformation point, the deformation resistance is low and the hot workability is excellent.

  First, the sintered billet 10 is heated to a temperature equal to or higher than the α + β region or the β transformation point, and is subjected to hot extrusion at that temperature to form an extruded material 20 having a stem portion 11 having a desired shape (FIG. 2b). In addition, 13 is an unpressed part.

  Next, it is reheated to a temperature equal to or higher than the α + β region or β transformation point, and the face portion 15 having a desired shape is formed by hot forging at that temperature to obtain a rough engine valve 30 (FIG. 2c).

  At this time, it is preferable that the heating temperature at the time of hot working of the stem portion 11 and the face portion 15 is in the range of 900 ° C. to 1200 ° C. for both. If the processing temperature is less than 900 ° C., it is difficult to sufficiently reduce the deformation resistance, which is not preferable. On the other hand, if the heating temperature exceeds 1200 ° C., the oxidation is intense, which may adversely affect the subsequent material properties and may cause fine cracks on the surface during hot working.

(5) Heat treatment step The heat treatment step is a step of heat-treating the engine valve rough material in order to coarsen the structure of the face portion and ensure desired creep characteristics. After forming the engine valve rough material 30 in the hot working process, the engine valve rough material 30 is heated in an inert gas atmosphere to a β transformation temperature + 50 ° C. or higher, for example, 1100 to 1200 ° C. × 0.5 to 2 hours. Thus, the β grains of the face portion 15 are coarsened. If the heating temperature is less than 1100 ° C., the β grains cannot be sufficiently coarsened. On the other hand, even if the temperature exceeds 1200 ° C., an effect corresponding to that cannot be obtained, which is uneconomical. A more preferable heat treatment temperature is 1150 to 1200 ° C. Further, if the heating time is less than 0.5 hours, the β grains cannot be uniformly coarsened. On the other hand, if it is longer than 2 hours, productivity is lowered, which is not preferable. A more preferable heat treatment time is 0.5 to 1 hour.

  By performing such a heat treatment, it is possible to remove distortion due to processing and to ensure excellent creep characteristics of the face portion 15 in a high temperature region. The reason why excellent creep characteristics can be obtained by subjecting the engine valve rough material 30 to heat treatment to coarse the old β grains and making it into a coarse acicular (α + β) structure is not necessarily clear, but is estimated as follows. The This is because the grain boundary slip is dominant in the creep characteristics in the temperature range to be used, so that in principle, it is effective to lower the stacking fault energy and increase the crystal grain size to increase the elastic constant.

  At this time, the stem portion 11 is also heated at the same time. However, since the reinforcing particles 200 exist in the stem portion 11, the coarsening of β grains is suppressed by the pinning effect. Therefore, the stem portion 11 can maintain high fatigue strength and wear resistance.

  FIG. 3 shows the microstructure of the axial cross section of the engine valve crude material after the heat treatment. (A) is a stem part, (b) is a face part. From (a), it can be seen that TiB particles, which are reinforcing particles, are oriented in the extrusion direction (horizontal direction) of the stem portion in the stem portion, and the matrix has a fine crystal structure. In addition, it can be seen from (b) that the face portion has a coarse structure, the particle size of β grains (former β phase) is about 300 μm, and α phase is precipitated in a coarse needle shape therein. .

  Then, the engine valve 40 (FIG. 1) having desired specifications such as dimensions can be obtained by subjecting the engine valve rough material 30 to a finishing process similar to the conventional one.

  As described above, according to the manufacturing method of the present invention, the portion having the reinforcing particles 200 in the stem portion 110 and no reinforcing particles in the face portion 150 without changing the conventional engine valve manufacturing line. Therefore, it is possible to easily provide the engine valve 40 having different reinforcing particle contents.

  Specific examples will be described below.

Example 1
As the raw material powder, commercially available hydrodehydrogenated Ti powder (# 100) and 36.9Al-24.9Sn-24.4Zr-6.2Nb-6.2Mo-1.4Si alloy powder (average particle size: 9 μm, numerical value) Is a mass% of the contained element.) And TiB ₂ powder (average particle size: 3 μm) as reinforcing particle powder were prepared. First, a predetermined amount of Ti powder and alloy powder were mixed and mixed well using an attritor to prepare a first material powder. Next, TiB ₂ powder was mixed with the first material powder so that the volume of the TiB particles after the reaction was 10% by volume, and mixed with an attritor to prepare a second material powder.

Then, the first material powder and the second material powder were sequentially filled into a cylindrical cavity (φ16 × 32 mm) of the mold, and the billet 10 composed of the face portion layer 10a and the stem portion layer 10b was formed. . The molding pressure here was 6 t / cm ² .

Next, the billet was heated at a heating rate of 12.5 ° C./min from room temperature to a sintering temperature of 1300 ° C. in a vacuum of 1 × 10 ⁻⁵ torr, held at 1300 ° C. for 4 hours, and then 1 ° C. The sintered billet was obtained by cooling at / min.

  Using this sintered billet, the stem portion 11 is formed by hot extrusion at 1150 ° C., then heated again to 1150 ° C. to form the face portion 15 by hot forging, 6 mm (stem diameter) × 35 mm A rough engine valve material 30 having a shape of (face diameter) × 120 mm (full length) was obtained.

  Subsequently, the engine valve crude material 30 was heated in an argon gas atmosphere at 1150 ° C. for 30 minutes to stabilize the creep characteristics of the face portion 15. Thereafter, predetermined processing was performed to obtain an engine valve of Example 1. The titanium alloy of the matrix was Ti-6Al-4Sn-4Zr-1Nb-1Mo-0.2Si (the numerical value is the mass% of the contained element).

(Example 2)
An engine valve of Example 2 was obtained in the same manner as in Example 1 except that the reinforcing particle powder blended in the second material powder was TiC powder.

(Comparative example)
Similar to the stem portion 11, the face portion 15 is a conventional engine valve in which reinforcing particles 200 are blended.

(An endurance test)
Each engine valve was mounted on a practical engine and a durability test was performed under the following conditions.
(1) Endurance test conditions a. Engine model: 6 cycle x 4 valve, 2000cc
b. Test conditions: 6400 rpm × 4/4 load, water temperature cycle 60 × 110 ° C.
c. Test time: 200 hours (2) Evaluation method The amount of change in protrusion of the engine valve (cupping + axial elongation: unit μm) before and after the 200 hour endurance test was measured.
(3) Results The amount of change in protrusion of the engine valve was 30 μm in both Example 1 and Example 2. However, in the comparative example which is a conventional engine valve, it was 75 μm, and it was confirmed that the amount of change in protrusion of the engine valve in the example was much smaller than 1/2 of that in the comparative example.

  In addition, this invention is not limited to said Example, You may change in the range which does not deviate from the summary of this invention. In the embodiment, the TiB particle amount is 10% by volume, but may be 5% by volume, for example. By setting the amount of TiB particles after the reaction to 5% by volume, the creep resistance can be further improved while ensuring fatigue characteristics and wear resistance.

  The engine valve of the present invention can be suitably used for an engine valve of an automobile engine, particularly an exhaust valve exposed to high-temperature gas.

1 is a schematic front view showing an engine valve of Example 1. FIG. It is a schematic diagram which shows the shape change of the billet in a hot working process. (A) is a sintered billet, (b) is after hot extrusion, and (c) is after hot forging. 1 is a photograph showing the structure of an engine valve of Example 1. (A) Stem portion, (b) Face portion. It is a graph which shows the relationship between the amount of TiB particles of a titanium alloy, and a fatigue characteristic. It is a graph which shows the relationship between the amount of TiB particles of a titanium alloy, and a creep characteristic.

Explanation of symbols

10: Billet 11, 110: Stem portion 15, 150: Face portion 30: Engine valve rough material 40: Engine valve 111: Cotter groove 200: Reinforcing particles

Claims (4)

An engine valve in which a stem portion and a face portion are integrally formed,
The stem portion is made of a sintered titanium-based heat-resistant alloy with a sintered titanium-based heat-resistant alloy as a matrix, and the face portion is made of the sintered titanium-based heat-resistant alloy as a matrix. An engine valve characterized by   2. The engine according to claim 1, wherein the stem portion has a needle-like (α + β) structure with a β particle size of 25 to 100 μm, and the face portion has a coarse needle-like (α + β) structure with a β particle size of 100 to 1000 μm. valve. A material powder preparation process for preparing a material powder by mixing raw material powders;
A powder filling step of filling the prepared material powder into a cavity of a mold;
A billet forming step of pressing the filled material powder to form a cylindrical billet;
A billet sintering step in which the formed billet is sintered to form a sintered billet;
A hot working step of hot working the sintered billet to form a rough engine valve comprising a stem portion and a face portion;
A heat treatment step of heating the engine valve crude material to coarsen the structure of the face part,
In the powder filling step, a region corresponding to the face portion of the cavity is filled with a first material powder composed of titanium powder and an alloy element powder that forms a matrix together with the titanium powder, and the stem portion of the cavity is filled. A corresponding region is filled with a second material powder further containing reinforcing particles dispersed and held in the matrix in the first material powder.   The method for manufacturing an engine valve according to claim 3, wherein the heat treatment step is a step of heating the engine valve crude material at 1100 to 1200 ° C for 0.5 to 2 hours in an inert gas atmosphere.